bad service/complaint

In August 2015 I booked my blackberry bold 9790 with <br> IMEI No. *** for software upgrade at an MTN store hemmingways East London. The phone was still under warranty and with no damages except for scratches. It stayed about 2-3 weeks then I was phoned to come collect it in September 2015, when I collected the phone I left the battery at home, I was told the software had been upgraded but the phone was not switched on and inspected upon receiving it as it was done when I took it in. When I got home I switched it on and noticed that the screen had been broken, you could only see when the phone has been switched on, the screen had flickering lines on it so I took it back to the store after a week as I was busy with my wife admitted in hospital. Then I received a call from Port Elizabeth repair center that the quotation of the phone is R2000+, I told them I was not prepared to pay that amount as my phone got damaged in-between the store and repair center,. I was then told to pay R330 for my phone to come back and I refused. MTN is keeping my phone and expect me to pay for its repairs or for it to be sent back meanwhile it got damaged in their possession. I need assistance.