In Wisconsin the legal drinking age is 21.    My question is can a undercover cop buy or supply a 20yr old escort with alcohol privately or publicly and still arrest her for prostitution?    I have a $100 bet on this.  
Thanks

to carry out their mission.   Police departments have their own list of rules.   Some allow officers to go all the way when busting an escort; others prohibit certain things, so to answer your question correctly, we'd have to know which police dept. is being considered.   Even then, these department regulations are often kept secret, so the only way to know for sure would be to join the department, or have legal cause to subpoena their regulations.

 
It's not a clear yes or no, as are most legal questions.

 
(still not a lawyer)

Assuming his department will back him up, and that includes breaking the very same laws he is being paid to enforce in many cases.

 
I doubt my opinion will be enough to settle your bet, but that has been my experience where it comes to what cops can and cannot do in order to bust a young woman for engaging in consensual behavior. As fishbro mentioned of course, it really depends on the jurisdiction, what will fly in Peoria might not fly in Pasadena.