I know of no "nice" way to fix this kind of problem.  The way data is stored in AP_TransactionPaymentHistory does not lend itself to correction through reversal / re-entry.  I'm curious to hear of how others deal with it. 

(DFDM corrections are not something I'll casually recommend to anyone).

I also cannot help in this situation but going forward you might want to consider enabling the field to restrict accounting date to one future period (Library Master > Company Maintenance > Preferences tab)

The new Document Date warnings are a good idea too.

Thanks for the suggestion!  I enabled this feature.