Section 6. If the claimant agency receives a written application from the debtor or nondebtor spouse contesting the claim upon which the proposed set-off is based, an administrative hearing shall be held by the claimant agency pursuant to the provisions of chapter thirty A; provided, however, that if the IV-D agency is the claimant agency, an administrative review shall be conducted in accordance with the provisions of section seventeen of chapter one hundred and nineteen A. The purpose of said hearing or administrative review shall be to determine whether the claim is valid and if so, whether the claimed sum asserted as due and owing is correct. No issue previously litigated by the parties in a court of competent jurisdiction and upon which a final decision on the merits has been rendered shall be considered at the hearing or administrative review. If the amount due is incorrect, a proper adjustment shall be made. Where a nondebtor spouse is contesting the portion of the refund attributable to his income, the nondebtor spouse shall have the burden of proving what portion of a refund transferred to a claimant agency, if any, is attributable to his income.