harvey duff, n. — Green’s Dictionary of Slang

harvey duff n.

[? the character Harvey Duff, an informer (a police sergeant disguised as a peasant) in The Shaughraun (1874), a play by Dion Boucicault (1822-90)]

money.

1916

Yorks Eve. Post 16 Oct. 5/4: Money in the army, as among racing men, is known as ‘harvey duff’.