Xiaomi India — service center not accepting their mistake

Address:

Ujjain, Madhya Pradesh, 456331

Today i went to mi service centre to replace my resmi note 3 battery. It cost 1350 but they only took 950 and didn't give the bill and they just took out the old battery casually which destroyed my phones display and now they are not accepting their mistake.
Now it has totally faded my display and also the mic of the phone during on calls are not working.
They did this repairing in front of me and they are not accepting me to pay the charges for the bill.