From What's the origin of the word "buck"?
Return of the Straight Dope
Curiously, "sawbuck," a ten-dollar bill, appears to be only indirectly related to buck. It got its name because some old ten-spots were denominated with Roman numeral Xs. The Xs looked like the X-shaped arms of the benches sawyers used to hold up logs for cutting. (Pictogrammic depiction: X-X.) The benches, which were vaguely similar to today's sawhorses, were called sawbucks.
, Cecil Adams
"The secret of life is, there ain't no secret, and you don't get your money back."