Green’s Dictionary of Slang

(US) a banknote, money.

1920		R.J. Brown ‘Thirty Days on the Island’ in Argosy 3 Jan. 🌐 Slip me a couple more of them mint tickets — just to show you’re on the level.
1929		M. Prenner ‘Sl. Terms for Money’ in AS IV:5 357: For bills as such, we may say [...] frogskins, mint leaves [...] or yellow boys.