Samsung — Samsung Grand 2 motherboard damage due to overheat.

we have purchased Samsung Galaxy Grand 2 in month of August 2014, we have warrant and extended warranty . so we are already in warranty. Suddenly our phone is dead and is not getting started. when we visited samsung customer care in nasik, they are saying that mother board is damaged due to overheat and is not covered under warranty. We are asked to pay 7000+ for the same. There is no other damage and we are still in warranty, still why should we pay for internal problem. Please help us