We use this expression to point out that somebody has to keep a promise that they made earlier.
You can use this phrase in ………… language.
A: I’m sorry I can’t do that report that I said I would do for you.
B: Oh come on! A deal’s a deal. I helped you with your presentation.
A: Okay, you’re right. I’ll do it right now.
A: Charlie, don’t forget you have to buy a round of drinks for the whole office because you lost that bet about the new boss.
B: Yes you’re right. A deal’s a deal. And I lost.
A: When are you going to invite us?
B: I’ll let you know tomorrow.