It's a deal. 一言为定。
这句话的含义和用法与汉语的一言为定相同。

【范例一】It's a deal. I'll pick you up tomorrow morning.
一言为定。我明早来接你。

【范例二】You can use my car this weekend. It's a deal.
你周末可以用我的车。一言为定。

【范例三】I'll let you know as soon as she comes back. It's a deal.
她一回来我就告诉你。一言为定。

【情景再现】
A: How about a game of tennis?
来比一场网球如何？

B: Okay. Loser has to take out the trash for a month.
好哇！输的人罚倒垃圾一个月。

A: It's a deal.
一言为定。

B: See you in the court.
咱们球场上见。