You can only reverse a transaction if it is unchangeable. An unchangeable transaction is identified by a grey zoom arrow (
) next to it. To make your transactions unchangeable, select the Transactions CAN'T be Changed; They Must be Reversed preference on the Security tab view of the Preferences window. You can change this preference at any time.

Customer and supplier payments
Note that if an early payment discount was applied to a sale or purchase, you also need to reverse the discount. For more information about how to find a transaction, see Finding a transaction.
Negative Stock