yen, n.² — Green’s Dictionary of Slang

yen n.²

[adoption of Jap. yen, a gold or silver coin orig. valued at approx. $1 in the mid-19C]

(US Und.) money; a dollar.

1908		B. Fisher A. Mutt in Blackbeard Compilation (1977) 151: The bankroll is now 73 yen to the good.
1928		Phila. Eve. Bulletin 5 Oct. 40/5: Here are a few more terms and definitions from the ‘Racket’ vocabulary: [...] ‘Yen,’ money; a dollar.