Drunk for free Puzzle

Its like a story.. Must sound great for all drinkers but do riddle their minds.
Once a man told his story to his friend about how he threw a party to two of his friends , and they all could drink as much as they wanted and went back home with all the cash they brought. That sounds crazy enough if someone didn’t sponsor them. But no one really paid for it. Lets see what they did !!

At both the Canadian and US borders their corresponding dollars are discounted by 10 cents i.e. A US dollar in Canada costs 90 Canadian cents and vice versa. The man realized its importance and went with his friends to the US side of the border. They all took drinks costing 10 US cents. Now he paid in US dollar and got back a Canadian dollar. Now he went to the Canadian side and again drank worth 10 Canadian cents, paid a Canadian dollar and received a US dollar. And thats how they continued whole night. At the end they didn’t loose anything but had a worth of party!!

How is that possible, who the hell paid for drink for these drunkards??
Whats the reason??