Eh, the kind of thing you’re asking about is essentially fiction. Not that murder for hire isn’t a thing, it’s just that it doesn’t work like anything you’ve read or seen in movies. It’s one of those things where if you aren’t part of a criminal enterprise, you aren’t going to be able to hire someone, and you’ll be hiring them from someone else in the same network.

So, in any semi realistic situation, there won’t be any arbitration or argument. You fail, you fuck up, you die. Or, I guess, turn state’s evidence, which is where what little about actual “contract” killing that’s known comes from. It isn’t like an actual contract.

Now, in fiction? Tons of options. Likely, you’d have whatever head of the crime network making the decision, maybe with other heads, maybe solo.

But, again, the term contract killing isn’t exactly about a contract. There’s not a formal arrangement involved. It’s contract in the meaning of hired.