Xiaomi India — mi max 2 display

I had given my mobile to your borivali west service centre. There was a scratch on the display which i had mentioned them. The fault was in the touch button and nothing relating to the display. They replaced my display along with the touch button and told me it is not covered under warranty. I had bought the mobile in sept 2017. Kindly let me know if the problem was in touch button then why i was charged rs 4017. They told me because there is scratch on the display we cannot cover you in warranty which according to me is wrong. Kindly reply