Actually, it's the other way around, right? You owe the plumber for work done. If he finished the job, you'd owe him. Unless you paid him upfront and he didn't finish, then he'd owe you. But just saying he owes you fifty makes it sound like he did the work and you still have his money.
The real issue here is who gets to decide the work is "finished." A plumber might say a job is done while you're still staring at a leaky pipe or a weird noise. That's where it gets messy. It's not just about who has the money, it's about whose definition of done matters.