Having owned a Hudsons Bay River Lot on the North Saskatchewan River and interested in the Company for many years, I agree with Lighter that there is no evidence of the use of "buck" in Canada as a unit of currency in the "old" days.
In addition to use of the pound sterling by the British colonists, it should be added that French Canada of the 17th-18th C. used French coinage until the English defeated them. The fur-trading companies issued tokens in trade, but these were never valued as a buck or anything equivalent to that.

The 1856 record of buck as a unit of currency (in California, at least) is intriguing; diaries and papers of West Coast settlers and travelers of that time may contain additional information.