This is a security feature to prevent staff from dishonest.
Scenario 1:
I issue an invoice to a customer at the sales counter for 100 pcs of apple at $1.00 each, I saved confirm the invoice, then print out the invoice to the customer and collect a total cash of $100.00
Later I edit the invoice item and change the qty to 50 pcs of apple, so I pocket the $50.00