Still entering manual company into Simply 2007 pro

Entering customer balances from year end....   accidently entered one twice...  what is the best way to reverse this...  enter a payment in the history?   How will this affect the retain earnings?

Enter a negative invoice amount and the problem will go away.

Sounds fair enough but what would be the result if you just entered a payment????  Is there a report that can be pulled to show these historical entries?

Yes you could just enter a payment. However that is not really what happened, that is, a payment was not made, an invoice was entered twice.

Run a detailed vendor aged report.