When running the Aged Accounts Payable report, generally only unpaid or overdue invoices will show. An invoice may show with a credit amount though in a certain situation.
In this scenario, the invoice was paid previously, the check was voided, and a new check was created.
A negative (or credit) amount may show on the report for an invoice when the void date of the first check is after the date of the new check.
For example if an invoice was originally paid in June, the check was voided with an September date, and then a new check was also cut with a June date, the void will not take go through until September. Thus, it's as if the invoice has been paid twice on the Aged Accounts Payable report until the date of the void. The negative (or credit) will show on the report until the date of the void is included in the report.