I tell the truth.
Every time I've done that I've been ripped off. So now you have made me extremely nervous and that's not a good start. And it makes me wonder if you are just like them. So unless you want to convince me otherwise...
Leave if incall.
Tell them I'll give them something at the door to cover their time & expense.
And
Btw, if you don't like that, it's proof I owe you nothing.
Optionally, you owe me for wasting my time.
A permanent thread in the deli which escorts & clients both agree is unacceptable behavior (20 min rush jobs, paying upon arrival) that would serve as a reference for new hobbyists as well as a reference or "official word" to point to?
Imho, paying upon arrival, I.e. paying someone before they have earned it, is a matter of dignity, which is too high a price to pay. It's supposed to be fun. Focus on that & all should be well.