they want me to pay for a phone under warranty

I have recently taken my iphone 7 plus to the istore, for repairs, because it just decided to stop working, no charging, not switching on. There device is still under warrantly so they told me they gonna replace it and they sent me a quote of R00.00 and so I accepted it, when I got to istore to pick it I was told I need to pay 6000, they made a mistake becoz the phone had a cracked display and scratches, they not telling me what is wrong with the phone, a phone cannot just stop working becoz of a crack display, the last time my phone fell it was 2 months ago and it was working just fine until when I was charging it just drained out. This phone has a problem they just don’t wannna take responsibility ive read many complains about the same problem online from iphone 7 plus. I realy trusted this brand, im so disappointed