Copying a record and paste it using the same table

I'm working on an invoicing system on Delphi5.
the invoice consist of mastertable and detailtable according to the law i can't delete an invoice.But deleting an invoice it means that i insert the same invoice under another invoice number. for example if i have an invoice with invoice nr. 5 if i delete it. it will remain under number 5 and it will take the invoice number 6 with negative amount so when i do vat statement they can see both one with positive amount and the other with negative one.