If you do an OTC sale to an anonymous customer, and then they return the goods and you want to do a refund, the procedure is as follows:

1. Call up the Customers|Account screen
2. Press the Select button and search for the OTC account and select it
3. Look through the transactions to find both the Invoice for the OTC sale you want to refund and its payment
4. Select each in turn and press the Reverse button to reverse the two transactions. This will create a credit to reverse the invoice and a refund to reverse the payment

Now do the physical reversal - ie put the item(s) back on the shelf, and give the customer back their money.

Note that the refund created by reversing the payment will have the same payment type as the original payment, and you will need to do the refund the same way.

Thus, if they paid by credit card, you will need to do a credit on their credit card. You should NOT just give them money from the till.