Dishonouring Warranty Claim

I have been using Sony handsets for the best part of my life. I took my perfectly in tact, Z3 compact to Cell C for a warranty repair of what was apparently the proximity sensor failure, I have subsequently received a quote of R3400. Sony claim that the screen is now lifting and there is a \drop mark"on the phone and that this \""could\"" be the reason for the sensor failure. They also claim the LCD is lifting which was definitely NOT an issue at the time the phone left my possession"