In cases involving a parent's obligation to support his or her child for a period before a child support action was filed (i.e., cases involveing claims for "retroactive child support" or "prior maintenance" ). a court may determine the amount of parent's obligation (a) by determining the amount of support that would have been required had the guidelines been applied at the beginning of the time period for which support is being sought, or (b) based on the parent's fair share of actual expenditures for the child's care. However, if a child's parents have executed a valid, unincorporated separation agreement that determined a parent's child supoort obligation for the period of time before the child support action was filed, the court shall not enter an order for retroactive child support or prior maintenance in an amount different than the amount required by the unincorporated separation agreement.