Your task is to devise a PoD (not messing with Union victory in the ACW, nor Italo-Prussian victory in 1866, nor Alaska Purchase, and preferentially not requiring an Anglo-American War) that would make the USA buy western Canada (Rupert's Land, Northwestern Territory, and Columbia, although Canada may keep those portions of Rupert's Land that later became northern Ontario and nortwestern Quebec IOTL) between the end of ACW and the formation of the Canadian Confederation.