You may need to add a custom refund amount on the fly from the point of sale. You can do this via the add custom refund amount option as per the below instructions.
From the point of sale click the action menu which will display several options.
Select the Add Custom Refund Amount option
This will add the line item to the checkout window.
Click on this custom refund line to bring up its details.
From here you can modify the amount and description. Click the check button to save your changes.
You can add more items to the transaction if needed then click the charge or refund button when ready to finalize the transaction.