Xiaomi India - Defective peice

I purchased a phone Redmi Note * from flipkart on **/**/****.. But the phone's screen started detaching from case.. When I went to service center first they said that u have bent your phone.. When I showed them proof that only one side is bent not the other. They started cooking other stories that you have overcharged your phone.. And they are not ready to give anything in written as well as at least diagonise the phone.. First when I asked they said that they can't open else the phone will not start again.. But when I asked that I'm ready to pay.. At once they opened and said that it will cost Rs. ****/- after discount Rs. ****/-.. But I'm sure that they are trying to fool.. I need my phone serviced under warranty else exchange I cannot bear the expense of your fault..