Hello Jittima,
I think this can be achievable in standard SAP, if I have not get it wrong. We have to take care the following configuration to make it possible.
- In VTFL or VTFA ( whatever billing is applicable for you) try to put "A = Purchase order number" in Assignment Number field. This will copy the Sales order PO number to Billing document header field "Assignment - VBRK-ZUONR".
- The VBRK-ZUONR will be copied to Accounting document when billing document released to accounting. This Assignment can be seen in the display accounting document or in FBL5N for the customer ledger.
Note: As this is a standard SAP configuration there will be minimum work to achieve this requirement. Still if you want the same PO no to be copied to each line items in the accounting document then you have to implement some enhancement to achieve the same.
Let me know your views.
Thanks,