Requested to pay over R5000 for an invisible windscreen chip

I have rented a 4X4 Toyota Hilux, on my return it had an invisible chip on the windsreen. I admit it might have been my fault. The chip was so small but I got an email that I will be invoiced for the full windscreen. The windscreen is not cracked. I then asked if they can give me an assessment report suggesting the windscreen cannot be fixed, they refused and told me the chip on the windscreen is growing legs. I asked how is that possible if when I dropped it off, there was just small chip? I then asked for inspection to see if the windscreen chip has since been tempered with or not but they are not allowing me to. I was willing to take responsibility for the damage but there is an ulterior motive from what I am experiencing. If I was able to add pictures on this review I would add so that you can see how I am requested to pay R5200.00 for a very invisible chip