@annewoodmayo five dollar bills are referred to as fins mostly on the East Coast, and especially in Newfoundland (lots of Irish descendants there). In fact, I believe the Newfoundlanders were the ones who called the five dollar bills fins and it spread from there. So I am not surprised East Coast USA also calls them fins.

A little history of the Abe:
The fiver got the nickname "fin" from a mistake by the Treasury Dept. in its initial printing in 1861. There was a typo in the motto which read "In Cod we trust." People took notice and started calling it a "cod" or a "fish". A few wiseacres would call it a "Gadus Morhua Linnaeus" per the cod's icthyological taxonomy, but when they said, "Ill take my change as two odds and a Gadus Morhua Linnaeus", only fish experts and Latin majors knew what the hell they were talking about, and that bright idea died a quick death. The bill was also referred to as an "odd" because five's an odd number. But, singles were also called "odds", which caused mass confusion. So, to keep things straight, if you wanted a five dollar bill as part of your change you'd say, "Give me an odd cod". At the same time as this was popular, "fin" was introduced and quickly became the predominant slang, simply because it's quicker to say, "I'll take a fin back", or "Slide me a fin and three odds". The typo was corrected in the next currency issue, but the nicknames stuck. A precious few "cods" survive and command princely sums among numismatists.

An aside:
The term "fishing for compliments" originated from the practice by gentlemens club patrons during the Civil War era of sliding a "fish" into the dancer's garter belt as a high complement for the girls they fancied most (the average tip for a bawdy dancer then was only an "odd", which was referred to as an "odd compliment": "She wasn't the best dancer, but he paid her the odd compliment anyway." This practice, in turn, spawned the idioms "give her the high fish", "slip her the cod", and "slip her the fin" but none of these stuck: "give 'em the high fish" already referred to feeding someone sarpa salpa, a species of bream that, when ingested, causes hallucinations; "slip her the cod" is a vulgarity for sexual intercourse; and "fin" is short for a Mickey Finn, a sedative used for spiking someone's drink with the intent to commit sexual assault. There was also a movement to replace "High fish" with "high five" which proved problematic because "high five" was used to describe the average length of an erect human penis. Over the years, that average was determined to be closer to six inches and high five was reintroduced to the slang lexicon and is still used today.