I'm not sure about this because I'm not an accountant, but in my view the cash registers are intended to be used stand alone. When you click "Register transactions" on the cash journal in the accounting dashboard what you see is the cash register, and you are supposed to insert the cash transactions there and then create the counterpart move lines from there. The drawback is if you need to print each transaction to PDF, which is only possible through the payments interface. In that case there are two options: add business logic that auto creates cash register lines on payment confirmation; or create a QWeb template in order to be able do generate a PDF from cash register lines. Or else, always register the cash transactions manually twice, first as payments, then as cash register lines.