Mi Mobile - Mi Phone - Gave faulty device and now at service center they say validity is void

I bought a Redmi Note * in August from Flipkart. The phone was working fine until * week(April **) before it stopped working (even before * year of use). I took it to the MI service center for repair. They checked and said the mother board will have to be replaced and I left the phone with them. When I went yesterday to the service center (April **) I was told that my phone had been tampered with, indicating that I had given it for repair outside. He indicated that it had some heat thrown on it which does not make sense as the phone looked like it had been scratched instead of heat on it and I am fairly certain they tampered with the phone before showing it to me.
I explained that I had never given it for repair and why would i give it outside and pay for repair when I have warranty for the phone and can get free service. They did not trust me after which I raised a complaint at Mi service center online. The service center person called me today(April **) and said that they trust the information given to them by their people and cannot trust me. Asking me to pay for the mother board change.
The phone has been taken care of and is in excellent condition with not even a scratch and even before * year it has stopped working and now they are not ready to consider it in warranty!!!!!!!