Purchase Order Amount Modification when Advance ( Prepayment) is Matched

Hello

When using Prepayment Match to Purchase order function, we are not able to modify the PO amount ( Unreserve> change amount). The error is  "QTY Ordered cannot be less than QTY Billed".

Prepayment is  Financed Amount on the Purchase order and Billed Amount is the Prepayment Application. So in the scenario where the prepayment application amount is less than the Financed amount, we shuold be able to reduce the PO amount?

Any suggestions/ref.docs will be appreciated.