A 5 dollar bill is called a sawbuck because of the sawbuck term that was a term derived from the resemblance of the carpentry tool to the Roman numeral X, is commonly used by interbank forex dealers to signify a transaction of $10 million U.S.
The 5 dollar bill also used to bear the Roman numeral X and that X resembles a sawbuck that was used and still used to cut wood with.
In the USA we sometimes call a sawbuck which we put logs on to cut a sawhorse as well.
In Carpentry a sawbuck is a holder for logs that we then use a chainsaw or hand saw to cut the logs on.
The Sawbuck held the logs up off the ground so that they could be cut.