To refund a credit card Terminal payment, you will have to refund the guest on the Terminal at your property and then open the guest's billing screen to post the Terminal refund.
Once you are on the billing screen of the guest, you can add a new Terminal Card Payment with the negative amount.
Click on Payments > Card Payment > Choose Terminal card payment:
To add a refund in the billing, you need to make sure the amount is in negative:
This will add the refund to the billing screen. Now the billing is balanced and you can close it: