Green’s Dictionary of Slang

1. (US) $1.

1934		Weseen Dict. Amer. Sl. 294: Banger A dollar; a silver dollar.
1941		C. Brackett & B. Wilder Ball of Fire [film script] The accepted vulgarism for a smackaroo is a dollar. That goes for a banger, a fish, a buck or a rug.

2. small change.

1963

A.S. Fleischman Venetian Blonde (2006) 253: He had taken my billfold and all I had left in my pocket were a few bangers. I spent them for booze.