Green’s Dictionary of Slang

(Aus.) any situation in which payment, esp. for alcohol, can be put off.

1891		Truth (Sydney) 29 Mar. 7/5: I must explain that getting in on the nod is the same as on the ‘never never’, ‘Freeman’s key’, ‘the ready.’.
1915		‘Alpha’ Reminiscences Goldfields i 51: He [...] usually went on the spree every second month, and while in that state lost more than he made, the house being freeman’s key for a time, until he again sobered up, after having had what he declared ‘a regular soaker.’ [AND].