If someone told you to buy some paper money, what would you say?

Those who fight back will answer: burn it for you?

It's a bit difficult to say that both sides laugh it off. It depends on each other's purpose. If it's a joke, just answer: OK, let's use it together.

If that person is trying to satirize you, this answer should be useless. You regard it as a joke, but he is not. If you don't want to be too stiff, just pretend that he farted and ignore it.