(一).It's a deal.就这么办吧。

This means "agree to your offer." Often the speaker is saying that he and another per. son have agreed on the cost of something that is being bought and sold.

原音重现：

A: If you pay me $7000, I'll sell you the car.
你出7000美元我就把车卖给你。
B: It is a deal. I need to go to the bank first.
成交，我先去下银行。

(二).A deal's a deal.这是约好的。
This means that a person can't change an agreement that was made. It is a way to say "You have to honor the agreement you made with me."

原音重现：

A: I don't want to buy the car now.
我现在不想买车了。
B： You must. A deal's a deal.
你一定得买，不是约好了吗？

美剧表达：
We have made the changes you asked for and it's a done deal.
我已经按你的要求改好了，就照约定的做吧