A 5 dollar bill is sometimes called a fin because finf is what the Yiddish name for five is.
So in Yiddish you would call the 5 dollar bill a fin which would be the name in Yiddish for five.
In the USA most people just call the 5 dollar bill a five or five dollars.
But when you're saying it in Yiddish you would call the 5 dollar bill a fin.
Also as to the reason a dollar is called a dollar is.
A dollar is called a dollar because the word dollar is derived from the Flemish or Low German word daler (in German taler or thaler) which is short for Joachimstaler, referring to a coin from the silver mines of Joachimstal, in Bohemia (now Jáchymov in the Czech Republic).
The term dollar was later applied to a coin used in the Spanish-American colonies and also in the British North American colonies at the time of the American War of Independence.
It was also then adopted as the name of the US currency unit in the late 18th century.
So that's where it comes from as to how the dollar got it's name.
When we refer to a dollar now it refers to 100 pennies, 4 quarters, etc or 1 dollar bill that is equal to just 1 dollar.