I've been thinking about this.

I probably shouldn't enter the retainer payments directly on the customer's current invoice—as you describe—because they were made quite some time ago (years ago, in fact).

Also, I deposited the cash that he sent at the time into my bank account. So I'll need to record them as deposits so I can reconcile them against my bank statement(s).

Given these complications, how might I best address the situation? I still will need to produce an ongoing statement of invoices/charges/credits against the retainer.

FYI I've used up about half of the retainer, in terms of time spent on the current project. I mention this to highlight the need to spread these transactions across multiple invoices.

Please advise.