Since the U.S. primacy of "buck" 'dollar' is well established by the Oxford English dictionary and all others sources that go into any detail, burden is upon those who claim a Canadian origin to present their evidence and explain why it is logically to be preferred.

Canadian evidence so far: an unsourced assertion on a website that explains nothing.

The earliest Canadian example of "buck" 'dollar' in the OED is from 1927. That's considerably later than the 1850s.

When "one man's fact is another man's opinion," at least one of those men is a nitwit. In this case the contenders are a public-relations website and the Oxford English Dictionary, which, incidentally, not only cites its sources but actually quotes them.

But, dude, who knows what's true and what isn't? Just because nobody landed on the green-cheese part of the moon doesn't mean most of it isn't green cheese. And I'm talking a whopping 99%.