Green’s Dictionary of Slang

eggplant n.1

(US black) a five-dollar bill.

D. Burley N.Y. Amsterdam Star-News 31 Jan. 16: [L]aying a righteous beg on all and sundry for a few eggplants.
[US]D. Burley Orig. Hbk of Harlem Jive 52: A five-spot is an eggplant.