Enhancement for VA01 transaction

Hi Techies,

I am stuck with a complex issue. The requirement is if payment term (VBKD-ZTERM) in sales order header found as 'U000 - Per

forma'. Then Pro forma invoice will be generated in background using BAPI (BAPI is executed for invoice creation).Delivery Block (VBAK-LIFSK) and Invoice Block (VBAK-FAKSK) will be updated after creation of pro forma invoice.

But now the issue is that in order to create a performa invoice, i need sales document number. And this will b generated only after i will click on save button. But the user wants to block sales order in this case.

How to achieve this? Any suggestions will be appreciated.

Regards,

Vinit