Hi Shilpa,
to have sales office as extra field in billing document header, you will need to extend the VBRK structure and use user exit USEREXIT_FILL_VBRK_VBRP (Module pool SAPLV60A, program RV60AFZC) to populate it from customer master when creating a new billing doc.
You will also need to decide how to use this new field.
If you want to display it the billing document screen (VF01-03 transactions), you will need to add it to one of the doc header subscreens (modification of standard SAP),
Using it in report, billing document output should not be a problem. Passing it to finance - will depend on where and how finance want to use the field.
Regards,
Dmitry