E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
cash or money generally. A suggested derivation may be mentioned. Quo = anything, and Quid pro quo means an equivalent generally. If now a person is offered anything on sale he might say, I have not a quid for your quo, an equivalent in cash.
Then, looking at the gold piece, she added, I guess you dont often get one of these quids.Liberty Review, June 9, 1894, p. 437.