In Acree v. Acree, 2 Va. App. 151, 342 S.E.2d 68 (1986), the Court held that "because of the unique facts" of that case, the father should be awarded credit for his non-conforming payments. 2 Va. App. at 152, 342 S.E.2d at 68. The Court found that the father had not altered the amount of support that he had paid, but rather he had altered the method of payment. The Court noted that "the agreement of the parties as carried out worked to the benefit of the child to the same degree as absolute conformity with the terms of the decree would have." Id. at 158, 342 S.E.2d at 72. In Acree, the Court made clear that where only the form or method of the child support payment, and not the amount, is modified by agreement of the parties, such modification may be proper and court approval may be obtained post facto. In such cases, the total amount of support to be paid has not been modified, and subsequent court approval of the change does not constitute an improper retroactive modification of the child support payment decreed. See Acree, 2 Va. App. at 157-58, 342 S.E.2d at 71; see also Code 20-108. In Acree v. Acree, 2 Va. App. 151, 342 S.E.2d 68 (1986), the Court found that equitable considerations may support the award of credits against child support payments otherwise due where the obligated parent has fulfilled the purpose and function of the award, albeit in a way that deviates from the express order of the court. Id. at 157, 342 S.E.2d at 71. Under the Acree holding, where the record affirmatively shows that a permanent change in the custody arrangement has been established, that the best interests of the child are served and that the agreed form of alternate payment satisfies the purpose and function of the support award, an impermissible retroactive modification of the award is not effected. Acree, 2 Va. App. at 157-58, 342 S.E.2d at 71-72; According to Acree, an impermissible retroactive modification does not occur when: (1) the parties have entered into an unequivocal agreement; (2) the change of custody is permanent; (3) the agreement has been fully performed; (4) enforcing the original child support obligations contained in the final decree would unjustly enrich one party; (5) enforcing the agreement would not adversely affect the child support award.