I do not have the solution at hand, I'm helping someone out who needs the answer to this question. (problem solving) So it's basically brainstorming. (I could not think of a appropiate solution, if any)
Take the billionaire out of the equation. You said that we are the good guys, hence no killing, so we could arrest the billionaire. Surely, somebody who has put such a large bounty on someone's head has done something that could warrant an arrest. Arresting the billionaire means he won't be able to pay the killer, which removes the motive for the potential killers. The chaos will die down and you will be able to transport him anywhere you like.
Alternatively, you could use decoy vans to transport Guy B. If we are willing to dedicate the resources of having 50 police officers around Guy B at all times, then decoy transport vans should be no issue. Wait until most of the potential killers have followed the other vans (the risk is reduced) and then send out another batch of vans, only one of which will contain Guy B. Or don't put Guy B in any of the vans. Dress him up in a disguise and have him travel in a prison employee's car.