You have a ledger transaction that you want to reverse and you are unable to reverse in the Purchase Invoice Register.
If the transaction was posted via the Debtors or Creditors transactions:
In Debtors/Creditors, select the same transaction type as being reversed from the ribbon.
Enter the relevant Customer or Supplier code, as well as the same date, period etc of the one to be reversed and then proceed to the detail area.
Enter negative values to match those in the original - ensuring that the same analysis record is used in each case.
Use Save & Close to confirm the posting.
Once a reversal has been posted, it is advisable to allocate this to the original, after which a new transaction can be posted with the correct information if necessary.