If your client does not currently owe a balance, you can add credit amounts to their account which can be put towards future outstanding balances.
If you'd like to issue a refund as credit, see Refunding an invoice.
To issue credit to a client:
Go to the client's profile.
Select the Payments tab at the top.
Click Add payment on the right.
Enter the amount you'd like to add as credit to the client's account.
Click Add credit.