It's a deal. 一言为定。
1. It's a deal. I'll pick you up tomorrow morning.
2. You can use my car this weekend. It's a deal.
3. I'll let you know as soon as she comes back. It's a deal.
A: How about a game of tennis?
B: Okay. Loser has to take out the trash for a month.
A: It's a deal.
B: See you in the court.