1、Its a deal ：一言为定 。

2、Thats settled then：一言为定

3、count it as settled：一言为定

例句：

“一言为定了。”外祖父说。

" That's settled," said the grandfather.

那样我就可以一言为定地告诉你了。

So that I may tell you decisively.

阿美：一言为定，你没有开玩笑，是吧？

May: That's great! You are not kidding, are you