PO for Rebate Agreement

Hi,

How can we consider the earlier PO's into rebate agreement. i.e when a rebate agreement is created later/after few days of agreement date, how can we include the purchase orders that were created earlier for rebate. For an example if the agreement is created by the end of month for a granter and if they want consider the PO's that was created for the granter in whole month, how can it be achieved.

Thank you