Nice to see that someone's tax dollars are going into government agents asking around on Gaming Forums on how to prevent the crime of the century.
Here's what I would do. Offer Two Billion Dollars to capture the Rich Man alive, and say that if the Rich Man or Guy B dies, there will be no money for anyone. Also say that teamwork is recommended, as anyone who directly assists capturing the Rich Man will each receive 2 billion dollars total. After they inevitably catch the Rich Man, and Guy B is secure, me and Guy B would book it out of the country overnight, without a trace. It'd be the ultimate scam, but it would be better than the ultimate murder.
First, have the president announce that nobody who kills Man B will enjoy the money b.c. they will be arrested and the money will be confiscated by the FBI. Then, hit him with a sleep dart from a distance. Helicopter cameras have notoriously narrow survelience and will miss the cause in this chaotic scene you are describing, and the spectacle of police surrounding him. Confusion will follow, nobody will be sure what happened, and the cops will have parametrics take him to the hospital, where you can scheme with the department to declare him dead, and have an undercover cop pose as the killer. The billionaire can be arrested by setting up the method of payment such that he is traceable.