If you want to do a refund on a different day, use the 'Refund' button in the top right corner while being in the Sales tab in POS.

Then either select the product that you want to refund, or use a Cash register item to enter the amount. Then press 'Pay >' in the top right corner and choose the tender type in which the refund should be made. If it's cash, just enter the amount in cash. If it's credit card, use 'Start refund' button in the credit card section. Be aware that unless tokens are enabled, you will need customer's credit card to process a refund.