Reusing the same invoice number

Our business users want to use the same invoice number for the same supplier and site.

Here is the business scenario.

While entering the invoice manually in the invoice workbench they make a mistake. If they realise the mistake before the invoice is accounted, they will prefix the invoice number with CAN- and then cancel the invoice. Because of this they can use the original invoice number once again.

But after accounting, the invoice number field is grayed out in the invoice workbench. the cannot follow the above work around process. If they cancel the accounted invoice, the accounted entries will be reversed, all the fields on the invoices will the grayed out and they can never use the invoice number again for the same supplier and site.