Make unused funds on release available as part of contract amount

We have a blanket PO with a few releases that are fully received but partially matched/paid.  The problem is the unused funds on the releases are counted against the contract total and are inflating the amount of the contract we've used.  Is there way to make the unused funds on the releases available as part of the contract total?  For example, when we cancel releases, the release Amount changes to the Matched amount and the funds are not held as part of the contract usage. We are not able to cancel these releases b/c their quantities have been fully received.