If someone hires you to collect a rare item, promising you money when you hand it over, what Jewish law bands keeping this item for yourself?

I think it would be banned under either the law against theft (though, does it count as theft if the person you were hired by doesn’t have it yet?)

Or it would be banned by violating contract law, as you agreed to hand it over to them for a set price.

Bonus points if there are any relevant quotes. I am writing a Graphic Novel with a major Jewish character, and I want to show faithfulness to their faith. Thank you.