Buck, the slang term for a dollar, almost certainly is a clipped form of buckskin; buckskins being used as units of commerce on the American frontier. We can see the semantic development in the following citations.
From James Buchanan’s 1824 Sketches of the History, Manners, and Customs of North American Indians:
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Each buck-skin one dollar.
Copyright 1997-2014, by David Wilton