Dear Sarma,
We also faced the same problem in our organization and we have resolved by using below scenario.
1. We are using Advance Shopping Notification (nothing but Inbound Delivery) over there we have appended structure to store Excise Duty for Vendor and PO values.
2. GR will be done as per ASN and part 1 will be posted as per the PO value.
3. During part 2 availment, we have done enhancement to bypass PO value and post with actual invoice value maintained at ASN.
(or)
After GR you can update the vendor invoice value along with MRP indicator active which is available under Miscellaneous and post.
Regards,
Karna J