The man paid for all the drinks. But, you say, he ended up with the same amount of money that he started with! However, as he transported Canadian dollars into Canada and US dollars into the US, he performed "economic work" by moving the currency to a location where it was in greater demand (and thus valued higher). The earnings from this work were spent on the drinks.

Note that he can only continue to do this until the Canadian bar runs out of US dollars, or the US bar runs out of Canadian dollars, i.e., until he runs out of "work" to do.

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