Open clhallege puzzle for ca students n all

In US, 10 Canadian dollars are equivalent to 9 US dollars and in Canada 10 US dollars are equal to 9 Canadian dollars (due to supply demand difference). A person who lives near the border, goes to a US pub and buys a US dollar worth of beer and gives a 10 US dollar note and gets back 10 Canadian dollar. Then he crosses border and buys a beer at Canadian pub and gives a 10 Canadian dollar note and gets back a 10 US dollar note!!