It had to do with when coinage would be cut into pices to make multiple smaller "bits" from the whole. A dollar coin could be divided in to 8 "bits". I still use the term but get that deer in the headlights look form people.
A "fin" was a 5 dollar bill, a "sawbuck" was a 10 dollar bill, since we rarely had them a "double sawbuck" was a 20 dollar bill. Never heard of anything bigger since they were so rare.