Thank you for your question.
The answer is that she cannot legally do that:
2014---Milot v. Milot, Va. Ct. of Appeals, Published, No. 0937-14-1
"The trial court erred by ordering Father to pay child support retroactive to a date prior to Mother’s motion for child support. Code of Virginia § 20-108.1(B) provides that “[l]iability for child support shall be determined retroactively from the date that the proceeding was commenced by the filing of an action with any court...” For a court to exercise the retroactive authorization provided in Code § 20-108.1(B), there must be, as a condition precedent, an existing and pending cause in a court of competent jurisdiction. Here, pursuant to a motion filed by Mother, the trial court entered a child support order making Father’s child support obligation retroactive to the day immediately following the dismissal of a prior divorce proceeding between the same parties in which the court had awarded Mother pendente lite child support. However, by dismissing the divorce proceeding, the court extinguished all of the pleadings and orders filed therein, including the pendente lite child support order. Thus, when the trial court entered its child support order, no prior final order existed compelling Father to pay child support. Accordingly, although the trial court had authority to order child support retroactive the date of Mother’s motion, it had no authority to order child support retroactive to the date the divorce action was dismissed."
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