I can share some information with processing a customer payment from a journal entry transaction, @JeanG.

A journal entry transaction will only show as an Unused Credits if you set the Accounts Receivable as Credit. If you set it as Debit, it'll show as a transaction that you need to process a payment (works like an invoice). Here's more information about adding, editing, reversing a journal entry: Create a journal entry in QuickBooks Desktop for Windows or Mac. 

Refer to the following article on how to clear up the net zero balances in your A/R Aging Summary report: Clear up customers with zero net balances in the Accounts Receivable Aging report. 

Here's how you can write off bad debt in QuickBooks Desktop to ensure your accounts receivable and net income stay up-to-date. It'll guide you with recording invoices that becomes uncollectible. 

Let me know if you need further information about journal entries or customer payments in QuickBooks Desktop. I'm always glad to help in any way I can. Have a great rest of the day!