Green’s Dictionary of Slang

(US) a $10 bill, $10 cash.

1834		Richmond (VA) Palladium 18 Jan. 4/2: Have you got ten dollars in small bills you will exchange for an X? [DA].
1836		N.Y. Herald 28 Apr. 2/2: In the morning finding that he was minus a watch and a X.
1849		G.G. Foster N.Y. in Slices 27: You see no money, except [...] when one of the players is ‘broke,’ he passes a V or an X up to the dealer, and receives its equivalent in buttons.
1853		J.G. Baldwin Flush Times of Alabama and Mississippi 11: All the V’s and X’s of ten years’ hard practice, went into that penful of ink.
1863		‘Miss Bailey’ in Donnybrook-Fair Comic Songster 61: I’ve got an X greenback.
1877		E.L. Wheeler Deadwood Dick in Beadle’s Half Dime Library I:1 83/3: He laid an X in the ruffian’s hands.
1893		W.K. Post Harvard Stories 295: Perhaps you had better lend me an X now.
1910		H.E. Lee ‘Tough Luck’ Variety Stage Eng. Plays 🌐 (Sport sitting [...] holding a ten dollar bill in hand) Gee! It’s a sure tip that my heirs are not going to fight over my fortune, and this X is my limit, it sounds better to say ten times one.
1916		J. Lait ‘Charlie the Wolf’ in Beef, Iron and Wine (1917) 40: It was a twenty-dollar bill, pinned to a note [...]. It was as follows — Here’s a dubble X for you.
1929		Hostetter & Beesley It’s a Racket! 242: ‘X’ — A ten dollar banknote.