Green’s Dictionary of Slang

(US) to hand over (money).

1907		Quad City Times (Davenport, IA) 17 Nov. 9/1: The upstate yoekel [sic] [...] unbuckling his mazume.
1914		Ade ‘The New Fable of the Father Who Jumped In’ in Ade’s Fables 84: Daughter alone could induce him to un buckle, and melt, and jar loose, and come across, and kick in, and sting the Check-Book.