They can't fix my phone for their own mistake.

I sent my phone for screen repair and when it came back they actually ruined my camera where as it was working perfectly fine before the screen repair, now they are asking me to pay more money on top of the service fee I paid. Otherwise they will sell my phone :-( . This is a day light ******* and it is unfair for me. Why should I pay for mtn repair's mistake? <br> I'm paying monthly a fee for insurance and this is the treatment I get when I have to claim