When creating cash invoices, you may notice it shows a Past Due amount.
This means there is an open cash invoice(s) that is not paid or not paid in full.
To find the invoice number(s), print Reports, Revenue, Open Invoice report. You can put in a long date range so it shows all open invoices. After you do the steps, if it still shows a balance then you didn't go back far enough and can run the report again with a longer date range.
Run the report by Customer and find the section for Sale, Cash to get to the invoice numbers.
Click on Accounting, Invoices. Search for the invoice(s), modify (or double click) and take payment.
Each invoice must be paid separately.