phone repair

My mother bought a Hisense phone at edgars Key west for my sister, however with 7 day the volume button was stuck or broken or whatever so it did a safe mode loading and got stuck there. The 7 days out of box warranty still app**** and the refused to change it then which was fine with us so they took it for 21 days repair yet they want us to pay for the repair while the manufacture's warranty is still valid.

How to do determine that something is negligence when the problem lay with the phone in the 1st place? We will not pay anything and just so you guys know a civil case against edcon is now been initiated well just after going through all the process that my lawyer will be sending me through. I look forward to all this process