Normally, once an item is posted, it is in the system.  Posting is the be-all & end-all.  It is kind of like writing with pen.  And let’s just say, there is a reason they invented whiteout.

Dynamics GP does have an exception to the posting rule in the form of Correcting Entries.  The one caveat is that only open year General Ledger entries can be corrected.

To make a correcting entry, select Transactions > Financial > Transactions.  Click the Correct button.  Select an Action of:

Back Out a Journal Entry
Or
Back Out a Journal Entry and Create a Correcting Entry

Select the appropriate journal entry to correct using the lookup feature & select OK.  Remember, only general journal entries can be reversed.  Review the Batch ID in the lookup window to determine if an entry is a general journal entry.