USD purchase invoice

We import goods from other country. We place order to vendor at USD X.
Than we pay 30% advance at price USD “a”
And balance at USD “a+/- 1”
Now when we pay custom duty, than custom depressed consider rate of USD as “a +/-2” , because custom dept declares rate of USD to INR every month irrespective of the current rate of Dollar.
Also they charge IGST, while we clear the consignment.
Can you explain is the process of purchase invoice entry, where in our assasable value is different than our purchase invoice value.

Please guide us.