Not sure it is a good idea. It might be more flexible when you issue an invoice and have to modify it, but first it would enable fraud, and second it would mean that you have to monitor incoming invoices to see if anything changed and account for that. I think it's better to ask people to just emit correct invoices: if you need to invoice a bit more you issue another one, if you need to invoice less you issue a credit note. If you can't do that you need to work on your business processes.