how to say It's a deal

Question

When you make an agreement with someone, how do you say, both formally and informally, "it's a deal."?

Like, formally "Do you agree to the contract?" "Yes. It's a deal."

Informally, "I'll make you food if you teach me Chinese." "Fine. It's a deal!"

Thanks

Answer 1

不见不散 is often used if you are making an appointment, especially for a dinner. 一言爲定 is more formal, and one may prefer 就这么定了 in oral conversation.

Answer 2

Oops, just remembered.

一言为定。 Yī yán wéi dìng

Answer 3

I agree with “一言为定”, also you can say "成交"

Answer 4

My additions:

You can also say 不见不散 when you want to have a appointment with others at some time in the future.