星级典句:
第一句: I want to change some RMB into American dollars.
我想把人民币兑换成美元。
A: Excuse me. Do you exchange foreign money?
请问你们兑换外币吗？
B: Yes. What kind of currency do you want?
是的，你要兑换什么货币？
A: I want to change some RMB into American dollars.
我想把人民币兑换成美元。

第二句: I want to exchange my dollars for pounds.
我想把美元换成英镑。
A: What can I do for you?
您需要什么帮助吗？
B: I want to exchange my dollars for pounds.
我想把美元兑换成英镑。
A: What's the amount you'd like to exchange?
您想要换多少?
B: I'd like to exchange $200.
我想兑换200美元。

其他表达法:
I'd like to convert the unused US dollars back into RMB.
我想把没用完的美元换成人民币。
I'd like to change some foreign currency.
我想换外币。