I never hand over $$ before the session when TOFTT for this exact reason. Show them you have the $$ as soon as you are in the door, then leave it in your pocket and politely say you will pay at the end because that's what your lawyer told you yada, yada in case this was a cop sting blah blah...you have been robbed before, blah blah....
If this is okay and SP agrees to the sessions, I will leave the $$ on top of my clothes after showering so they can see it's there which puts them at ease.
else If they have issue with this arrangement, then its a red flag and better walk.
I will say 75% of time when TOFTT, the SPs are okay with this. It comes down to your appearance, dress and demeanor. I make sure to look clean cut and don't dress or act like a rough hoodlum.