cannot fix a phone under warranty due to dents

I purchased an iphone 6 plus on the 09th of April 2015 R13000.00 cash at istore Fourways, within 8 months that was December my phone started freezing, i took it to istore Centurion so the technicians can help fix it,they booked it in where it was supposed to be fixed together with the receipt of the phone assuring me that i am supposed to get a brand new phone should they fail to fix it because my phone is still under warranty and i bought it cash, after few days Carmen Van Aarde started sending me emails saying they are unable to fix the phone because they don't have resources to fix it and i will get a new phone if only i give them 6000 because my phone has dents outside the phone, i told them i don't have that kind of money and i don't want them to fix dents i want them to fix their software problem, they refused saying i should pay them 6000 because my warranty doesn't cover dents, please assist me because when i took that phone back i wanted them to fix their software problem not the dents or scratches made on my screens phone, paying 6000 is like getting another phone and my phone is still under warranty, i cannot afford it, help me please.as im talking my phone is still ther