You always allocate a customer refund to one of the following:
If you allocated a supplier refund, delete it before you edit one of these transactions. You can't save supplier refunds on account.
If you use the Cash Accounting VAT scheme and the refund is on a submitted VAT Return, you can't delete it.
Use the From and To boxes to select the correct date range. Select More, and change the Type to Supplier Refund.
You've now deleted the refund. Any transactions you allocated to the refund now show as unpaid.