{dissenting). The plaintiff, a painting contractor, entered into a contract with the defendant to paint a building being constructed by the defendant, a general construction contractor. The dispute concerns the amount of work plaintiff agreed to perform under the contract. The plaintiff claims the contract was agreed upon orally; the defendant asserts the contract is evidenced by a writing not signed by the plaintiff.
The majority affirm the trial judge’s ruling that the parol evidence rule precluded him from considering, as trier of fact, the testimony, provisionally taken, of plaintiff’s witnesses. This testimony was offered to show that a purchase order sent by the defendant to the plaintiff was not accepted by the plaintiff and, for that reason, never became a contract, that the true contract was made orally. In my opinion testimony is admissible for that purpose.
It is plaintiff’s testimonially supported claim1 that the parties entered into an oral contract several *505weeks before the defendant sent the purchase order, that the purchase order contained a provision requiring the plaintiff to do painting* work not required *506under tbe earlier oral contract, that upon receipt of the purchase order, plaintiff called the defendant and defendant thereupon acknowledged the inconsistency and stated that the plaintiff should perform in accordance with the terms of the oral contract.
The plaintiff performed the alleged oral contract. The defendant contends, however, that the purchase order is the contract and that the plaintiff is required to render the additional performance therein provided. Plaintiff brought this action for nonpayment *507of its invoices; the defense is that the plaintiff did not perform all the required work.
Under the parol evidence rule, a writing that has been adopted as the complete integration of the terms of a contract may not be varied by evidence of negotiations or understandings that preceded its adoption.2 The testimony offered by the plaintiff did not, however, seek to vary the terms of the purchase order.3 Rather, it was the plaintiff’s contention that the defendant’s purchase order, neither signed nor accepted in writing by the plaintiff, never became a contract.
Just as parol evidence maybe offered to prove that the signature on a writing is a forgery and to show (as in this case) that a purchase order or other document was sent by one party to another and that the recipient later rendered performance required by the document, so, too, parol evidence may be introduced to show that an unsigned document was not accepted, or that it was orally modified before it was orally accepted or the performance was rendered.
In this case the facts that a purchase order was sent by the defendant to the plaintiff and that the plaintiff rendered performance required by the purchase order, which facts defendant claims establish that the purchase order became a contract, were all established by parol; on the same principle which permits those facts to be proved testimonially, the plaintiff had the right to introduce testimony that before any performance was rendered the terms of *508the purchase order were orally rejected or, by mutual oral agreement of the parties, modified.
In Canvasser Custom Builders, Inc. v. Seskin (1969), 18 Mich App 606, the applicable principle was clearly stated by our Court (p 611):
“[I]t is an equally well-entrenched rule that parol evidence may be introduced for purposes of proving to the Court which of the two alleged agreements was the actual contract of the parties. This seemingly paradoxical situation is explained in 3 Corbin on Contracts, § 582, p 450, which states:
“ ‘The very testimony that the “parol evidence rule” is supposed to exclude is frequently, if not always, necessary before the court can determine that the parties have agreed upon the writing as a complete and accurate statement of terms. The evidence that the rule seems to exclude must sometimes be heard and weighed before it can be excluded by the rule.’
“See, also, 9 Wigmore, Evidence, § 2430(2).”4
This controlling principle was also applied in Tuuk v. Andersen (1969), 21 Mich App 1. There, as here, the defendant relied on a writing not signed by the plaintiff and plaintiff’s rendition of performance required by the writing after its receipt. We held that the trial judge ruled correctly when he allowed the *509plaintiff to testify that before he rendered performance the parties had entered into a separate oral agreement concerning the subject matter of the writing.
It is entirely true that if the plaintiff in this case had rendered performance required by the purchase order without objecting to its provisions plaintiff might be found thereby to have accepted the provisions of the purchase order.5 By failing to object the plaintiff might be deemed to have agreed that the purchase order superseded any earlier oral contract. But the plaintiff’s representative testified — without contradiction by the defendant — that he did object, and that when he did so the defendant’s representative agreed that the objected-to provision of the purchase order would not supersede the alleged earlier oral contract, and, in that regard, the oral contract would govern. It is that testimony, no reference to which is made in the majority opinion, which distinguishes this case from Cohen v. Jackoboice (1894), 101 Mich 409, on which the majority rely.
It is, of course, a factual question to be resolved by the trier of fact whether an oral contract was entered into before, and reaffirmed during conversations that took place after, receipt by the plaintiff of the purchase order.
The testimony of the plaintiff’s representative6 was corroborated by one of the defendant’s former *510employees. Tbe defendant did not produce at tbe trial tbe person in its employ witb whom plaintiff’s representative said be bad tbe conversations.7
Tbe defendant does not seek to defend tbe trial judge’s decision adopting tbe purchase order as tbe contract on tbe ground that it was based on a finding of fact, and that such finding is not clearly erroneous. Tbe defendant does not assert that tbe judg'e considered, but chose not to believe or for other reasons rejected, tbe testimony of plaintiff’s witnesses and found as a fact that tbe purchase order became tbe contract. Tbe defendant’s brief is replete witb statements that the trial judge properly refused to consider tbe testimony of the plaintiff’s witnesses.8
That tbe trial judge did not disbelieve tbe plaintiff’s witnesses is attested to by bis finding that an oral contract bad in fact been entered into before the defendant’s purchase order was sent.9 He stated, *511however, that the purchase order was “a written contract or at least a contract which was a subsequent contract reduced to writing which would be the binding contract between the parties unless there was fraud, mistake, or some other method of holding it ineffective.” He added that since the “more recent offer to employ the plaintiff was in writing” and it was specific, the writing controlled. The judge, thus, took the position that a document bearing a date later than the date on which an oral contract was in fact entered into must, as a matter of law, be deemed to supersede the oral contract, even though the party to be charged never signed the document.
A document cannot prove itself.10 There is no rule of law which required the plaintiff to reject in writing the defendant’s purchase order.11 Ñor is there any rule of law which required the plaintiff to send the defendant a confirmatory written memorandum after the conversation alleged to have taken place following receipt by the plaintiff of the defendant’s purchase order.
It is entirely understandable that the plaintiff and the defendant who, according to the plaintiff, had done business for over twenty years, much of it pursuant to oral contracts, might also on this occasion have entered into an oral contract. A document not signed by the plaintiff has no greater sanctity than the oral contract the trial judge found was entered into before the purchase order was sent.
As just mentioned, the trial judge found that the parties entered into an oral contract which obligated the plaintiff to paint the building, and that this contract was agreed upon before the purchase order was *512sent by the defendant to the plaintiff. The judge found further that under the oral contract the work to be done was “the painting other than the partition painting.” (See footnote 9.) Thus, when the defendant subsequently sent the purchase order requiring the plaintiff to paint the partitions, it was attempting to change the oral contract. This the defendant could not do without the plaintiff’s agreement to the change.12
“An exact and unconditional acceptance of an offer is not afterwards turned into a conditional acceptance so as to invalidate the contract by an improper interpretation of the terms of this contract' by one of the parties or by his attempt to alter the terms of the contract in some respect.” 1 Corbin on Contracts, § 85, p 367.
“The so-called confirmation or acceptance was hut an offer to change the terms of the agreement and was not accepted, by defendant.” Goldsmith v. Stiglitz (1924), 228 Mich 255, 258..
This ease should be remanded for the taking of additional testimony, and for findings of fact on the issue whether the conversation which the plaintiff alleges took place following, its receipt of ’the purchase order regarding the disputed painting work did in fact take place, and for further proceedings after that factual issue is resolved.
Benjamin Hexham, an executive of the plaintiff, testified that the plaintiff and the defendant had done business together over a *505period of 20 years. Some of the transactions were covered by writings, others were entirely oral.
Hexham met the defendant’s representative, Milton Dickson, to discuss painting the building.
Over objection, Hexham testified that he looked over the plans and that he and Dickson agreed that the plaintiff would do the work for $5,252, and that it was also agreed that plaintiff’s work would not include the painting of 2,000 feet of interior partitions which, according to a note attached to the plans, was part of the work.
Two weeks to a month later, plaintiff received the defendant’s purchase order number 1802 dated April 29, 1965. The purchase order stated that for a price of $5,225 the plaintiff would do all tho painting and decorating work provided for in plans and specifications that had been prepared by the architect; the plans and specifications were specifically identified by architect job number and page numbers.
Hexham said that upon receipt of the purchase order he called Dickson and reminded him that interior partitions were not included and that Dickson responded, “ ‘That’s right.’ Well, I can’t remember exact words, that’s right, okay, go ahead, get the work done. That is all. "We want to get the building up. Something of that nature.”
(The record does not explain the discrepancy between Hex-ham’s testimony that the purchase price was $5,252 and the purchase order price of $5,225. The parties have offered no explanation and apparently do not regard it of any importance. The explanation may be simply that the court reporter transposed the last two digits.)
Subsequently, on May 12, 1965, the defendant sent the plaintiff copies of the specifications. The plaintiff does not claim that there were any further conversations regarding the partitions following the receipt of the specifications. The specifications provided that they cover “all work throughout the building, the interior and exterior usually painted or finished to make a complete job, in every respeet.”
On February 28, 1966, the plaintiff billed the defendant for $5,225; plaintiff’s invoice stated that the work had been done “in accordance to [sic] plans and specifications” and referred to defendant’s purchase order number 1802. (The fact that the plaintiff’s invoice to the defendant mentioned the purchase order and stated that the work was done in accordance with plans and specifications is, indeed, some evidence negating the plaintiff’s elaim that the purchase order did not evidence the contract between tho plaintiff and the defendant. But it is merely evidence to be weighed together with the testimony regarding the conversations alleged to have taken place before and after the sending of the purchase order.)
The plaintiff did paint some of the interior partitions. The plaintiff claimed that this work was dono pursuant to a separate agreement evidenced by the plaintiff’s bid in its letter of June 9, 1966 and the defendant’s responding purchase order of August 2, 1966. The plaintiff’s letter stated, and the purchase order repeated, unit X>riees, i.e., so much per square foot and so much per door, but no quantities were mentioned. The purchase order stated that these unit prices were “for additional work.”
*506The plaintiff contends that tbe defendant’s agreement to pay for tbe partition work supports tbe plaintiff’s claim that it was not required to do that work under tbe April 29, 1965 purchase order. Tbe defendant responds that tbe plans provided only for a certain minimum quantity of partition painting and that tbe June 9, 1966 letter and tbe August 2, 1966 purchase order dealt with additional work beyond that specified in tbe plans and specifications.
Tbe plans prepared by tbe architect stated that they covered 1500 lineal feet of wall in tenant areas and 320 lineal feet of wall between tenants plus 140 doors. As previously mentioned, Hexham testified that there was a note attached to tbe copy of the plans which he looked at when be originally bid the job which stated that the job included painting 2000 lineal feet of partitions.
It would, therefore, appear that the specifications themselves are of no importance in resolving the dispute. Whatever was meant by the words in the specifications stating that they cover “all work throughout the building, the interior and exterior usually painted or finished to make a complete job, in every respect,” those words were not meant to require the plaintiff to paint partitions beyond the 1,860 (1,500 plus 360) lineal feet referred to in the plans or the 2,000 lineal feet referred to in the note attached to the plans. This the defendant appears to eoneede when it states that there was work to be done that was not covered by the plans but which the plaintiff was not obliged to do under the April 29, 1965, purchase order because it was “additional work.”
George Lerehen, a former employee of the defendant, testified that he was in charge of coordinating the subcontractors employed by the defendant to construct the building. He testified that Dickson told him that he had entered into a contract with the plaintiff for the painting of the building and that the contract did not include the painting of partitions, that “we will have to take that up at a later date.” He testified further that subsequently he advised Dickson that the plaintiff would not paint the partitions until an agreement had been reached regarding price and that, pursuant to Dickson’s instructions, he thereupon solicited a price, following which the plaintiff sent the letter of June 9, 1966 and the defendant responded by sending the purchase order of August 2, 1966.
Milton Dickson did not testify. There was no testimony contradicting the testimony of Benjamin Hexham or George Lerehen.
3 Corbin on Contracts, § 573, p 357.
Cf. Brady v. Central Excavators, Inc. (1947), 316 Mich 594, 608 et seq., and Massachusetts Bonding & Insurance Co. v. Transamerican Freight Lines, Inc. (1938), 286 Mich 179, recognizing that evidence offered to show that a writing purporting to represent the contract of the parties was not intended by them to constitute a complete integration of all their understandings may be admitted because such evidence does not seek to vary the terms of the writing but rather to establish that it does not constitute a complete integration of the entire contract.
How could the law be otherwise? Suppose a seller and a buyer have been doing business for many years and the buyer reorders by mailing a new purchase order. Suppose further that the seller, upon receipt of the purchase order, telephones the buyer and tells him that quantities of the desired commodity are in short supply and, accordingly, he can only ship a lesser quantity and must do so at a higher price. And that thereupon an oral contract is reached for delivery of a lesser quantity at a higher price, and the goods are in fact shipped and accepted, but, as in this case, no writing is sent after the telephone conversation before shipment and acceptance. Under the majority opinion, the seller in such a case not only could not recover the higher price, but also could be required to respond in damages for failing to deliver the quantity stated in the purchase order at the lower price there stated. That simply is not the law. See 3 Corbin on Contracts, § 577, pp 385-388: cf. Sams v. Feldman (1955), 342 Mich 10, 13.
See, 1 Corbin on Contracts, § 70, p 288.
The trial judge, of course, was not required to credit Hexham’s testimony. Hexham was an interested witness; on that ground alone the judge might properly have rejected his testimony. 5 Callaghan, Mieh Pleading & Practice (2d ed), § 37.205, pp 557, 558; MCLA § 600.2158 (Stat Ann 1962 Rev § 27A.2158) ; Goppelt v. Burgess (1902), 132 Mich 28, 30; Ball-Barnhart-Futman Co. v. Lane (1903), 135 Mich 275.
There is no reason, however, to believe that the trial judge did not believe Hexham and Lerehen. Indeed, the judge must have concluded that Hexham was a credible witness; unless he reached that conclusion, he eould not have found, as he did, that an oral contract *510had been entered, into before the purchase order was sent. (See footnote 9.)
The trial judge did not rule that the testimony of the plaintiff’s witnesses, taken provisionally over the defendant’s objection, was inadmissible until after both parties had rested their eases.
The defendant’s claim that the purchase order is the contract is contradicted not only by the testimony of plaintiff’s witnesses, but also by the permissible negative inference whieh may be drawn based on the defendant’s failure to call Milton Diekson as a witness to contradict the testimony of Benjamin Hexham and George Lerchon. It is a well-established principle that if a party has it within his power to call a witness and fails to do so, the trier of fact maj properly infer that if the missing witness were called his evidence would support the opposing party. See Brandt v. C. F. Smith & Co. (1928), 242 Mich 217, 222, and Prudential Insurance Company of America v. Cusick (1963), 369 Mich 269, 285 (per Black, J., in an opinion signed by three other justices).
The following is an excerpt from the defendant’s brief:
“The above purchase order being in writing, Judge Moore was correct in refusing to permit the introduction of oral testimony to vary its terms as to the painting of tenant partitions. Oral testimony could only be introduced in the event of fraud or mistake, neither of which were presented, alleged, proven or raised by this appeal.”
Similar statements are to be found in three other places in the defendant’s brief filed with our Court.
The trial judge stated: “Now at this point, I would think that the plaintiff had established a contract orally to do the painting *511other than the partition painting, later to be constructed because of the uncertainty of the tenancy partitions. And an oral contract, followed by performance, would be a binding contract.”
3 Corbin on Contracts, § 582, p 448; 9 Wigmore on Evidence, § 2430, pp 102-104; Tuuk v. Andersen (1969), 21 Mich App 1.
Tuuk v. Andersen, supra.
See Shepherd Hardwood Products Co. v. Gorham Brothers Company (1923), 225 Mich 457, 463, 464; American Parts Co., Inc., v. American Arbitration Association (1968), 8 Mich App 156, 174; Gondola v. Yaksich (1966), 3 Mich App 676, 679; Peck v. Miller (1878), 39 Mich 594, 596; The Frederick Raff Company v. Murphy (1929), 110 Conn 234 (147 A 709) ; Cook v. Kerr (Mo App, 1917), 192 SW 466; Baker v. Dawson (1958), 216 Md 478 (141 A2d 157); 17 CJS, Contracts, § 50, p 707.