Green’s Dictionary of Slang

(US) a coin; thus castings, cash.

1843		G.W. Harris ‘Quarter Racing in Tennessee’ Spirit of the Times (N.Y.) XIII Mar. in Inge (1967) 20: I’ve got the fastest horse in these United States, at any distance, and lots of castings to back my judgment.
1851		‘Wars Yure Hoss?’ in T.A. Burke Polly Peablossom’s Wedding 41: A substantial farmer, who, by years of toil, had accumulated a tolerable pretty pile of castings.