Green’s Dictionary of Slang

(UK police/und.) £5 notes.

1923		J.C. Goodwin ‘Criminal Sl.’ in Sidelights on Criminal Matters 165: We touched for some fuine, a red toy, some spark props, a bit of wedge, and a half century in brass.
1950		P. Hoskins No Hiding Place! 190/1: Fuine. £notes.