I'll pay you back next week.

我下周把钱还给你。

《牛津词典》

Some day I'll pay you back for this!

总有一天我会为此报复你的！

《柯林斯英汉双解大词典》

Would you mind paying for me? I'll pay you back later.

你先给我垫上，以后再还你。

《新英汉大辞典》

I promise I'll pay you back tomorrow.

我保证明天就还你。

I didn't 实战 - SpeakingMax英语口语达人

if your friend asked to borrow $100 from you, ... you would probably not say-- you might actually put interest on this person, so you could say, alright, I'll do it with 5% interest, pay me back in a year.

你的朋友向你借100美元，你可能不会说-,你可能会对他收取利息，因此你没准会说，好吧，我要收5%的利息，一年之后连本带利还我。

耶鲁公开课 - 金融市场课程节选