Difference between Invoice value & custom assessable value

I am importer , import invoice value is 10 lacs in inr, but custom department said goods value is 20 lacs, Igst charged on amount of Rs 20 lacs, but my actual shipment value is only 10 lacs which was sent through bank to foreign suppliers.
Now i want to know whats is the right accounting treatment. if i sold the goods then my output liability is less than my itc. Whats is my purchase value 10 lacs or 20 lacs,
pls explain in details