OPINION
FORSBERG, Judge.Appellant Beltrami County brought this action to obtain reimbursement and ongoing support for assistance furnished for the support of respondent Donald E. Anderson’s child. The matter was heard by an administrative law judge (AU) under the procedure established by Minn.Stat. § 518.551, subd. 10 (1990). The county argues the AU erred in ruling that the reservation of child support in the dissolution decree precluded reimbursement for past assistance furnished under Minn.Stat. § 256.87, subd. 1 (1990). We reverse and remand.
FACTS
A decree dissolving the marriage of respondent and Janet’ L. Anderson was entered on September 17, 1986. Respondent was awarded physical custody of the parties’ two older children, and Janet Anderson was awarded physical custody of the youngest child. The trial court found that both parties were unemployed and receiving AFDC. The court reserved child support “until such time as the parties become employed and/or receiving other sources of income.”
In March 1988, respondent obtained full-time employment. Janet Anderson continued to receive an AFDC grant in the amount of $437 per month. No party moved to set a child support obligation for respondent until the county commenced this action under section 256.87 in April 1990.
*721The AU ordered respondent to pay ongoing support of $270 per month and to reimburse the county in the amount of $1,080 for assistance provided since May 1, 1990. The county moved for amended findings or a new trial, arguing it was entitled to reimbursement for assistance furnished during the two years immediately preceding this action. The AU denied the motion, concluding that section 256.87, subd. 1, limited AFDC reimbursement to child support payments accrued under the dissolution decree. The AU reasoned that because the decree reserved child support, there could be no arrearages and no right to reimbursement.
ISSUE
Did the AU err in refusing to order AFDC reimbursement for the two-year period prior to the commencement of the county’s action because the dissolution decree reserved child support?
ANALYSIS
It is within the trial court’s discretion to determine whether reimbursement should be ordered under section 256.87 and what amount should be reimbursed. State, ex rel. Region VIII North Welfare v. Evans, 402 N.W.2d 158, 161 (Minn.App.1987). The outcome in this case, however, results from the AU’s interpretation of section 256.87, subd. 1. Statutory construction is a question of law and is subject to de novo review on appeal. Doe v. Minnesota State Bd. of Medical Examiners, 435 N.W.2d 45, 48 (Minn.1989).
The parent has the primary responsibility to support the child, and the county should only be expected to contribute to the extent that the parent is unable. Nicollet County v. Larson, 421 N.W.2d 717, 720 (Minn.1988). This policy is served by the statutory mechanism allowing the county to recover past public assistance provided to the child or the child’s caretaker from the noncustodial parent.
A parent of a child is liable for the amount of assistance furnished under sections 256.72 to 256.87 to and for the benefit of the child, including any assistance furnished for the benefit of the caretaker of the child, which the parent has had the ability to pay. Ability to pay must be determined according to chapter 518. The parent’s liability is limited to the amount of the assistance furnished during the two years immediately preceding the commencement of the action, except that where child support has been previously ordered, the state or county agency providing the assistance, as as-signee of the obligee, shall be entitled to judgments for child support payments accruing within ten years preceding the date of the commencement of the action up to the full amount of assistance furnished.
Minn.Stat. § 256.87, subd. 1 (1990) (emphasis added).
The AU determined that reimbursement for assistance furnished during the two years prior to commencement of the action was not available to the county because the reservation of support in the original dissolution decree was an “order” for support. We disagree. The express reservation of a child support obligation does not constitute a determination of child support. Bennyhoff v. Bennyhoff, 406 N.W.2d 92, 94 (Minn.App.1987). Accordingly, where support is reserved in the original decree, the subsequent establishment of a support obligation is treated as an initial matter rather than a modification of a prior support order. Bennyhoff, 406 N.W.2d at 94.
The letter of the law shall not be disregarded when the words of the law and their application to an existing situation are clear and free from all ambiguity. Minn. Stat. § 645.16 (1990). Under the clear language of section 256.87, subd. 1, the exception to the parent's liability for actual assistance furnished in the two years prior to the action does not apply here because child support was reserved, not “previously ordered.”
DECISION
The AU erred in concluding that the county was precluded from obtaining reim*722bursement for assistance furnished during the two years immediately preceding the reimbursement action because of the terms of a prior dissolution decree. Respondent is liable for assistance furnished during that period according to his ability to pay. We reverse and remand for reconsideration of the county’s action for reimbursement under section 256.87, subd. 1.
Reversed and remanded.