In addition to issuing a full refund, you are able to issue also a partial refund if needed. Please note, that your payment provider needs to support partial refunds for this option to be available.
In order to issue a partial refund, you can look up the user's account in the Piano dashboard, and under the Transactions tab click on the transaction that you'd like to refund partially.
After clicking the button Refund, you'll be shown a second screen, where you can enter the amount of the refund.
Once this partial refund is issued, another refund for this transaction is not possible via Piano.
If needed, the rest of the payment would need to be refunded directly via your payment provider's account/dashboard.