To make changes to the manual lines
You can do all the intended operations (add, edit, or delete an existing line) simultaneously if the invoice is in draft status. If the invoice is approved, you must revert the status to Draft.
If the invoice is not in Draft status, you can still see the existing ad hoc line items in read-only mode.
The changes made to the manual lines are updated on the invoice record after the invoice is approved. The Total Fee Amount field on the invoice now includes the fee amount resulting from the changes you have made during the 'Edit' operation.