Allow correction of supplier invoice
It may happen that we receive a correction invoice from a supplier. For the same reason that we do correction for customer invoice, we should do the same for supplier. But the problem is that the design is not suit for the workflow because we may receive from the supplier one credit note to correct multiple invoices etc.
So I think it will be better to have a flag on the supplier invoice line to mark the line as a correction. This way the amount is taken to update the unit price but not the quantity.
This design can not be used for customer invoice because the anglo-saxon accounting will be wrong, we must keep using the account_invoice_correction
module.