This has a been a problem for a long time. Most Great Plains users are aware of it but some folks never quite get it.
When you void an AP payment there are two possible steps:
1) Void the payment (Void Historical Transaction)
2) Void the voucher/invoice (Void Open Transaction) if you don't want to pay this again.
In both cases the screens look very similar. What's not clear, is that the VOID DATE and POSTING DATE can be changed by the user.
Many times I've seen users try to post voids to closed periods or even worse, post to prior periods that should have been closed but weren't. Now the financial statements change. YIKES!
So remember, you change the posting and void dates to whatever is appropriate, it's just not obvious. I wish MS would fix this. It's a simple fix. I've done modifier changes to highlight these fields for clients who just didn't get it. You shouldn't have to customize this, how about a UI fix here guys!