Whoever made that joke about breaking up with his girlfriend for four dollars, I will definitely adopt it.

I think so, too. If she doesn't pay back the money, don't talk about it yet, and don't break up with him yet. Go out another day and say you are thirsty and have no money. Let's see what she can do. She buys you 2 yuan of water, so you can be good to her. If she wants to buy/kloc-0 yuan of water, you have to do it first, at least you can lose one yuan. If she won't buy it for you, so can you.