The only way Manager can correlate payments and sales invoices, whether pre- or post, is through Accounts receivable, because that is where the customer’s account actually is. Accounts receivable is a control account, made up of individual customer subaccounts. It is an asset account, so is not included in any sales income account and does not distort your income (assuming you are using accrual based accounting, which you should be if you are taking deposits against future work).
The sales invoice is what posts revenue to the income account, balanced by the account receivable. You should not raise the sales invoice this year, but next year. A request for a deposit, if necessary, can be handled with a sales quote, appropriately renamed. This has no financial impact. See these Guides:
https://forum.manager.io/t/customer-deposits-and-advances/7093
https://forum.manager.io/t/creating-sales-quotes/7238
While posting a deposit to your Customer prepayments account is not wrong from a financial perspective, it is unnecessary and/or creates more work. Eventually, you will have to transfer it to Accounts receivable by journal entry, or you will miss out on the ability to allocate to specific invoices and have all transactions appear on a Customer Statement. If you post the deposit to Accounts receivable, everything can be automatic. Until July 28, 2016, you couldn’t do this, because you could not post a deposit directly to Customer credits. Then Customer credits was combined with Accounts receivable, making separate liability accounts for deposits unnecessary.