Hey Mailman
I agree with you. It's sneaky, dirty, whatever. I would not be applauding to your sense of morality here in any way.
Yet, the dude was allegedly placing ads looking for someone to suck his dick. I assume, he was not advertising that he was a working boi, escort, etc. When you responded, he came up with the ca$h issue. OK, not a major crime but still miles away from a truthful and honest approach.
The fact that the big dicked dude does not stand for an example of honesty does not make it right for you to go into the little game you have concocted there. Yet, I do not see any real harm done to anyone. If you want to suck him desperately on the false pretext as much as he wants to get sucked by other men for cash on the false pretext, too... go for it. Both of you may have some fun.
KD