For credits on account you would typically enter these as prepayments from the client detail screen.
You would still need to deposit to a bank account, however you can mark the date for the payment to pre-date your financial year. This will obviously affect your bank balance so when you setup your opening bank balance, simply adjust it by the amount of the credit.
Once you save that credit, it will be visible on the client detail screen and it will also be available under the “Log a payment” dialogue box for future invoices.
Hope that makes sense, please let me know if you’d like any further clarification.