AR Refund with no outstanding balance on invoice

Hello,

How do I record a cash refund to a customer when there is no outstanding balance on the invoice.  The refund was already issued in cash for amounts they paid, but it wasn't an overpayment or anything. The balance owing on the invoice is 0, but we still gave the money back (so they would just need to pay the invoice at a later date).

The refund module is not showing invoice document numbers unless it had amounts outstanding. 

Thanks