I wouldn't be paying them £100 to fix that. Is the dent from levering off the spoiler with a screwdriver or something?
I can understand your concerns and would feel exactly the same. I also think u should be praised in how uv handled this- creating a new profile and showing pics which cannot let us identify ur car to protect the trader not urself. If I was the trader I would think, fair play the guys not been an arse and gone public, he's been very amicable il take this one on the chin and fix it
Let's hope that's the outcome