In Acumatica, when you reverse an allocation, the system automatically posts it to the current period, which is why your reversal went into September instead of August.  So, you can’t directly change the reversal date to a past month like August if the period is closed. One option is to reopen the August period, post the reversal, and then close it again. If reopening is not possible, you can create a manual adjustment to fix the allocation in August and make another adjustment in September to balance things out. You could also create Project Transactions to fix the allocation for the right dates. From this way, you can make sure your entries are corrected in both periods, even if Acumatica doesn't let you directly change the reversal date.