If you're close to the border you'll probably get american change. In a real Canadian city: no, you would get Canadian money back. And coins are traded at par no matter the exchange rate.
It's actually pretty comical the amount of Americans that come to Canada and expect to only deal in American currency. Like there are people that genuinely believe we use American money up here.
Do people seriously believe that? That's crazy. I generally pay with my credit card that has no foreign transaction fees so I don't have to deal with stores trying to rip me off on the exchange rate