that's a deal 一言为定

“一言为定”，汉语成语，意思是一句话说定了，不再更改。比喻说话算数，决不反悔。可以翻译为“that's settled then，a promise is a promise或that's a deal”。

例句：

好，就这样办，一言为定。

All right, that's the way to do it. It's settled then.

一言为定，决不反悔。

I give you my word and I'll never go back on it.