Hi Vishwanath,
As indicated by Lakshmipathi sir, you can very well have a logic in print program of your Proforma Invoice output: The printout can show the title as "Invoice". Please discuss with your Abaper and do the changes. Ofcourse, before doing this you have to get confirmation from business.
BTW, Is this requirement is from India customs? I never came across such situation, our client used to take proforma invoice printouts.
Regards, Sai Krishna.