Green’s Dictionary of Slang

promise land n.

a promise.

1935		Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 8: Promised land: Pay on a future date.
1959		F. Norman in Encounter n.d. in Norman’s London (1969) 59: I can’t afford to get captured because I’ve already got six cons (convictions) and the last time I went up the steps (the sessions) the judge stuck me in promise land for a neves (told me I would get seven years next time).