Can a police officer authorize the sale of a pharmacy's drugs to a suspected criminal without paying for the drugs first?
Does this violate the United States or State Constitutions?
There is no problem authorizing the pharmacy to conduct the sale. Directing or ordering the sale might be a problem, but it's the pharmacy's objection that matters (not the alleged criminal's). Usually, the pharmacy has an interest in cooperating with these activities and does not object. Hence, no problem.
It seems to me that shoplifting would be a good analogy. A pharmacy has an interest in stopping theft. The police may survey a suspect while giving the pharmacy instruction to allow shoplifting to occur. The pharmacy cooperates with this because it's entirely in their interest to provide the police with the evidence they need.
Now, if the cops demanded a few bottles of oxy to sell during a sting, yeah, a pharmacy could certainly challenge the reasonableness of that seizure. The pharmacy, not the accused.*
You mention a reverse sting in the comments. There are plenty of examples of judges using fairly strong language against police who go too far, but the law is pretty new. In other words, reverse stings are getting popular and cases with certain facts need to make their way through various courts in order to be decided in favor of the accused.
*You might be on to something with this question, along with the other one about outrageous conduct and the case where cops manufactured their own drugs. Ordering a unwilling pharmacy to provide drugs for a sting might be as outrageous as manufacturing drugs, but that is an extremely fact-specific question.
A pharmacy can refuse a police order to subvert their normal and customary policies with respect to drug purchases by suspected criminals.
i.e. if a pharmacy would decline to fill a prescription because of their normal policy, they are within their rights to refuse to do it merely because the police asked them to. Alternatively, if their normal and customary policies would have allowed the purchase, then they might indeed participate with the police.