Green’s Dictionary of Slang

mint sauce n.

money.

1825		‘The Ugly Old Widow of Estramadura’ Universal Songster I 2: There’s too much mint-sauce to refuse her.
1830		Egan Finish to the Adventures of Tom and Jerry (1889) 53: I [...] hope that he gets lots of mint-sauce* [* Mint-Sauce. One of the numerous cant terms for money].
1867		J. Greenwood Unsentimental Journeys 230: ‘Mint sauce’ (as that horribly vulgar and slangy B.B. terms money).