Matching to Invoices

 11.5.10.02

Hi all

When ever we matched a prepayment to a Purchase Order and paid the prepayment, when the final payment is being paid and the prepayment is applied to it, the system reverse the portion that was matched to the prepayment initially and the Po  becomes opened. As a result what ever amount that was matched on the prepayment is now available to be matched again on the PO.

How can I prevent this from happen? I know that if I dont match the prepayment and matched the final invoice it will be ok. I also try to apply the prepayment to the final invoice then try to matched it back to the final invoice, but i notice that once you apply the prepayment to the invoice the matched button is grayed out.  But what other options is available out there.