(concurring specially).
Although I agree with the end result of the majority opinion, it fails to explain how or why res judicata applies in Issue III.
This is a continuing battle between the seller and the buyer of the Tri-State business. Buyer agreed to purchase the TriStáte business and pay Seller three things:
1. Opdahl was to receive 13,750 ounces of silver, valued at $275,000.
2. In addition, Hoff agreed to pay a bank note owed by Tri-State before May 1, 1980.
3.Finally, the proposal provided that Hoff was to make monthly payments over a period of ten years, beginning September 1, 1980. The proposal indicated that these payments would cover one-half the equity of TriState, plus certain other equipment.
Tri-State Ref and Inv. Co., Inc. v. Apaloosa Co., 431 N.W.2d 311, 312 (S.D.1988) (Tri-State I). Tri-State I specifically addressed and resolved parts 1 and 2, except only for the remand for redetermination of damages for breach of lease and recalculation of prejudgment interest. Id. at 317. However, the monthly payments required in part 3 were not litigated in Tri-State I. Consequently, res judicata will bar a claim for the payments due under part 3 only if the claim should have been properly raised as a counterclaim in Tri-State I.
On November 28, 1988, after remand of Tri-State I, Seller’s successor (Sioux II) sued Buyer and Buyer’s successors on part 3 claiming “that a total of $223,339 was to be paid in installments commencing September 1, 1980, but [Buyer] failed to pay $121,060 due on the contract.” Sioux II argues that this is not a compulsory counterclaim that should have been raised in Tri-State I because the installment payments were current at the commencement of that lawsuit. Sioux II is partially correct in its argument because Tri-State I was commenced in 1984 and the installment payments did not stop until June of 1986.
SDCL 15-6-13(a) specifies that:
A pleading shall state as a counterclaim any claim which at the time of serving the pleading the pleader has against any opposing party,
(emphasis added). We have interpreted this rule to mean that “a party need not assert a counterclaim that has not matured at the time he serves his pleading.” Staab v. Skoglund, 89 S.D. 470, 478, 234 N.W.2d 45, 49 (1975). Moreover, res judicata may not bar a claim that is not a compulsory counterclaim because it arose after the *779pleadings were served. Dillard v. Security Pacific Brokers, Inc., 835 F.2d 607 (5th Cir.1988).
Although Tri-State I was commenced in 1984, we must look to the dates when the pleadings were served to determine whether a counterclaim had matured and was compulsory. An answer to a complaint is part of the pleadings. SDCL 15-6-7(a). Opdahl and Sioux Enterprises served an answer to Tri-State’s amended complaint on September 3, 1986. Tri-State had ceased making the installment payments three months earlier, and the amended complaint implied Tri-State had no intention of resuming payment. Tri-State’s claim that Seller breached the purchase and sale agreement by converting over $150,-000 of Tri-State’s silver was sufficient notice that the entire agreement was in dispute and Seller should raise all of its claims against Tri-State. Therefore, the claim against Tri-State had matured when the answer to the amended complaint was served, making the claim a compulsory counterclaim now barred by res judicata. SDCL 15-6-13(a).
I am authorized to state that Justice MORGAN and Justice MILLER join this special writing.