Grenade, 
you can't do anything. without trying anything, pay them off. 
I've gone through similar incidents, I tried everything I could reading ppl's comments off forums and internet, 
but in practical,it doesn't work that way. They think you made them look like liars and they try their heart out
to make you prove guilty. Ultimately, it's a waste of time and effort, and you end up paying feeling like loser.

So, just pay it now, without those hiccups. Don't listen to anybody here who's suggesting you to do this and that.
They've nothing to lose, it's you, who'll be penetrated. So, don't argue with them and pay immediately. 

I'm telling you this because, there is no reason for them to trick you.  I've lived in my apartment for 5 years and 
there is no sign of dents,stains anywhere. Now these ppl are coming to me changing things for free. 

I'm pretty much sure, you didn't take care of your apartment that well or may be there are some poorly handled
indoor conditions.