I have a customer who has already paid in 2018. He is now requesting an invoice for that payment. How can I go back and create an invoice without creating a new amount owed and without causing problems for that payment that is already in there?
Delete the deposit and create a Sales Receipt instead - where you select the bank account to where it was deposited, so the bank balance is not affected - and Save and Send it by email or print to snail mail.
You only use an Invoice when the sale date is different to the payment date i.e terms provided