I see no means to add credit to a customer account.
Suppose there is a problem and as a courtesy you want to add a one time credit to their account so they will owe less for next, or current billing.
I see one could probably do a “refill” card and log in as the client and enter it, but that is not very intuitive.
Also it would be great to add it so it is shown in the invoice as a credit with a note in the description line.