Gst on import (declared price vs assesable price)

DEAR SIR,

       PL. ASSUME WE IMPORT ITEM A AT 1000 USD AND CUSTOMS ASSESS THE SAME ITEM AT 1500 USD FOR CUSTOMS DUTY AND IGST. 

       IF THE PRICE IS 1000 USD THAN OUR CALCULATION IS AS BELOW
1000 USD X 65 = 65000 (DECLARED INDIAN VALUE)
CUSTOMS DUTY 10% = 6500
CUSTOMS EDU CESS 2% = 130
HIGHER EDU CESS 1% = 65
IGST @ 12% = 7800

SO OUR COST IS 
65000 + 6500 + 130 + 65 + CLEARING & FORWARDING EXP (ASSUME 1000 INR)= 72695 (COST)
IGST: 7800 WHICH WE CAN COLLECT ON SALES.

BUT WHAT IF CUSTOMS ASSESED THE GOODS @ 1500 USD

CALCULATION IS AS BELOW
1500 USD X 65 = 97500 (DECLARED INDIAN VALUE IS 65000 ONLY)
CUSTOMS DUTY 10% = 9750
CUSTOMS EDU CESS 2% = 195
HIGHER EDU CESS 1% = 97.5
IGST @ 12% = 11700

SO OUR COST IS 
65000 + 9750 + 195 + 97.5  + CLEARING & FORWARDING EXP (ASSUME 1000 INR)= 76042.5 (COST)
IGST: 11700 
(EXTRA GST PAID IS 11700 - 7800 = 3900)

NOW PL. ADVISE US HOW TO AVAIL BENEFIT OF 3900 IGST EXTRA PAID IN CUSTOMS) CAN WE USE THIS IGST IN THE SAME ITEM A OR WE CAN USE THE EXTRA IGST IN ITEM NO B ALSO WITH HS CODE FOR ITEM A : 48025450 AND ITEM B: 48114900? 

WHAT IF THE EXTRA IGST REMAINS IN OUR ELECTRONIC LEDGER UNUSED. CAN WE CLAIM BACK THE UNUSED IGST? IF YES IS THERE ANY TIME PERIOD FOR CLAIMING? 

HOPE YOU UNDERSTAND OUR QUERY AND REPLY US POSITIVELY 

RGDS,
CHETAN RANKA
+91-98842-54160.