Clearly this video shows every cop in the country also demonstrates what they can do to you under the letter of the law. Congrats. You win the thread.
Cops have many restrictions under the law and the accused has many privileges. Remember, we live in a system that presumes inocence. Some bad apples cannot be applied to everyone.
Cops do not have absolute power.
You do. Don't want to talk to a cop? You don't have to. Not under an arrest? The cop has no right to hold you against your will. And it's not illegal in any shape or way to carry with you any ammount of cash.
Now, carrying a large ammount of cash and a scale with you
could give the cop reasonable cause to hold you for questioning or further investigation up-to an including holding in a cell pending investigating/bail as there's a precedent set that drug dealers will commonly carry large sums of cash with them and possibly even a scale.
To Catch a Predator is also legally in the clear because the accused in those cases not only break the law by simply
talking about sex with what they believe to be a minor (and that's a key point regardless of what the reality is) but they also travel distances, sometimes even crossing state-lines, under the intent to have sex with a minor. It's hard to rationalize that in favor of the accused if any of it is unreasonable, manufactured, or false then that's for a jury to decide.
But, consider that undercover officers all of the time work as drug dealer king-pins, prostitutes and other members of the criminal element to sniff out the dirt of our society. These internet stings are no different than an undercover operation.