How to refund a Paid Prepayment invoice

Hi,

We need to know the steps to do a refund for a  paid prepayment invoice.

Scenario is as follows.

Paid Prepayment Balance     100

Supplier Invoice Balance          80 (This amount is applied to the prepayment)

Currently user is entering a miscellaneous receipt in Receivables module for a prepayment account.So accounting entries does not have any impact.

But after the application of prepayment balance of 20 prepayment is remaining  in AP and that 20 is available as a prepayment which user can erroneously apply it to a standard invoice.

Please let us know a standard method to do this scenario.

Thanks in advance.