Reverse posted prepayment and credit memos

SUGGESTED

We had a customer use a third party to pay for invoices. They asked for a dummy credit we could send them so they could zero out invoices on their account. 

To do this, I was instructed to create a prepayment (as a positive amount) and then a credit memo for the same amount. The idea was that we wanted to zero out the two with each other, so the customer couldn't come back and use it later, or have it sitting in the aged invoice report and never get used. 

We used the prepayment to wash the credit memo out, but this has since created a negative balance in cost of goods sold (the CM item code used was "*M" - miscellaneous). 

Is there a way to reverse posted CM's and prepayments? Or an easier way to reconcile the cost of goods sold balance? 

Input is appreciated.