A way around this problem. I have added an extra line to the previous invoice (or create a Purchase invoice) with the overpayment. Then allocate the money so that the balance for the supplier equals zero. then raise a credit note from the invoice you allocated the overpayment (remove any additional lines on the credit note and you should then be able to refund to the correct bank account (You cannot do this credit note from the bank account where the money has come into if you have added an additional line, but it would be possible if you the overpayment has been allocated to an invoice with only the overpayment).