This article shows you how to create a client credit.
1) Go to Billing > Credits/Debits > Credit Memos
2) Click "New Credit"
3) Input amount, client, and description then click "Create"
*Credit is automatically assigned to the oldest invoice(s); you can uncheck the box(es) and assign to whatever invoice preferred.
*You have two "Types" when creating a credit. "Apply to invoice(s)" is the default and will directly apply the credit amount to a certain invoice; while "Retain as available credit" will save the credit for you to apply later from the payments/balances page.