It is common sense to not pay anyone until you are in the room with the girl stood in front of you. Why did he not ring the girl to confirm this was how she wanted to be paid and then walkedif she said yes
What that bloke tried to do to you sounds like a variation of the Soho walk ups scam of the bloke outside taking payment.
But this sounds like collusion with the girl as she is a 'sub' who is owned by her master and only does his bidding according to a set of rules. So they have woven the con into a BDSM scenario to make it sound more plausible.
When she takes the booking she obviously says I'm only having sex with you because my master is ordering me to be a slut and he wants the cash as compensation no doubt under her rules of 'ownership'.
Sneaky bastards because as you say it's common sense not to pay until you see the girl in front of you, but some guys obviously buy into this as part of the sub slave set up. Because her feedback is zero some people don't even bother to click on it so won't see the negatives. As you say think with the big brain not the little one!
Another good candidate for the Prostitute Blacklist I'd think.