Recording a Refund against a PO Matched Prepayment Invoice

Summary

When a Prepayment Invoice is matched to a Supply Chain Purchase Order, it is only available to liquidate against that PO. How can I clear the Refunded Prepayment balance?

Content

When you issue a Prepayment to a Supplier and they refund a portion of the amount, one solution is to create a Standard Invoice with a distribution to cash and apply the Prepayment. But this only works if the Prepayment is not otherwise tied up (i.e. not matched to a Purchase Order).

When a Prepayment invoice is matched to a Purchase Order, it is not only restricted to just that Purchase Order, it is even restricted at line level.  So if a supplier does refund a portion, I'm at a loss how I can relieve the open Prepayment balance.  Any ideas?