I promise I'll pay you back. 我发誓，我肯定会还你的。

You can pay me back whenever you like. 你想什么时候还，就什么时候还。

I pay you back all the money you invested. 我会一分不差地把你投资的钱还给你。

You don't have to pay me back. 你不需要还我钱。

Do you promise to pay me back? 你发誓能还我钱吗？

You have my word. 我保证。