Dear Abhijeet ,
if you want to pass this billing number to sales order and you have to fetch the data means follow the below logic .
Pass billing number to VBFA_VBELN and VBFA_VBTYP_N = C then fetch VBFA_VBELN ( sales order number ) and pass in VBAK as per my post .
or
If you want to fetch directly form Billing document please follow Ramalingam N solution .
Regards,
C.B Reddy.