Wordorigins.org

A sawbuck is a ten-dollar bill. These bills used to have two large roman numeral tens ("X") on their reverse side, resembling the scaffold for a sawbuck or sawhorse. From The Knickerbocker magazine of 1850:

Send me the two double “saw-bucks.”

Sawbuck is not known to be related to buck, meaning a dollar, but the possibility cannot be dismissed. They certainly influenced one another in any case.

(Source:Oxford English Dictionary, 2nd Edition)