Hi Diwakar,
That is a question for SD Billing, not for GTS.
As you've already noticed, GTS takes care of it by logging an error message. In the ERP system (in my humble opinion), you should take care of it by having proper process control, so that users do not needlessly create proforma invoices. If users don't know what they are doing, you are going to end up with far worse situations than this 
.
Regards,
Dave