Green’s Dictionary of Slang

dog n.3

[ety. unknown]

(W.I.) a small copper or silver coin.

[WI]S.A. Mathews Willshire Squeeze 31: Pepper and limes, and salt sometimes, / Cost four dogs more. [...] A bit is the twelfth-part of a dollar, and 6 dogs is a bit.
[WI]S.A. Mathews ‘Sayings of the Negroes’ in Willshire Squeeze 78: A black dog (a penny half-penny) buys trouble, and a hundred pound can’t clear you of it.