Green’s Dictionary of Slang

portrait of Madison n.

(US) a $5000 bill.

1953		R. Chandler Long Good-Bye 73: Not with a portrait of Madison in it. A portrait of Madison is a $5,000 bill.
1972		K. Bonfiglioli Don’t Point That Thing at Me (1991) 150: It contained [...] a portrait of Madison. The portrait, of course, was on a thousand dollar [sic] bill.