Id know what category this is but it is a brain teaser I can't solve, please help?

A while back, when in canada, 90 american cents equaled a canadian dollar and in america, 90 canadian cents equaled an american dollor, a man (bob) buys ten cents worth of beer from a canadian bar, pays with his american dollar, and gets an american dollar back. He then crosses over the boarder to america and buys ten cents worth of beer. He pays with his canadian dollar and gets a canadian dollar. He continues to do this until he ends up drunk, and has the exact same dollar in his pocket. Who pays for the beer?