Bingowings said:
What I like about it is he's back to being a bounty hunter and like in ESB he gets paid twice for the same job.

The exposition needn't be explicit, all we need to know is the Rebels have paid Fett for tracking the Emperor and finding the Death Star and Vader has paid him for making sure the Rebels arrive at the party on time (Dave just did it like that to get the idea across).

"I hope it's worth the price we paid" takes on a different meaning.

I just don't see it as believable that the rebels would trust a bounty hunter who has just been involved in delivering Han to Jabba the Hutt, and it's clear where his allegiances lie - if you were to argue that a bounty hunter works for whoever pays him the credits, then it would be obvious to the rebels that he'd go back to the Empire (who can pay him more) and double cross them, especially since he's had dealings with them before.

I imagine the rebels would be extremely careful about any bounty hunters they hired. Being a small, vulnerable group, they'd need to be 100% certain where the person's allegiances lay before hiring them.