Green’s Dictionary of Slang

job n.¹

a guinea, a pound.

a.1674		‘Of the Budge’ Head Canting Academy (1674) 12: And when that we have filed him / Perhaps of half a Job.
c.1698		B.E. Dict. Canting Crew n.p.: Job, a Guinea, Twenty Shillings, or a Piece.
1708		J. Hall Memoirs (1714) 12: Job a Pound.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
1741		Canting Academy, or the Pedlar’s-French Dict. 112: Twenty Shillings A Job.
1754		Scoundrel’s Dict. 14: A Guiny or Job – Huskin-lour.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
a.1790		H.T. Potter New Dict. Cant (1795).
1809		G. Andrewes Dict. Sl. and Cant.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1835		G. Kent Modern Flash Dict.
1848		Flash Dict. in Sinks of London Laid Open.
c.1850		Duncombe New and Improved Flash Dict. n.p.: Job or Jobey a guinea.

In phrases

half-a-job (n.)

half a guinea.

c.1698		B.E. Dict. Canting Crew n.p.: Half a Job, c. half a Guinea, Ten shillings, half a Piece, or an Angel.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
1741		Canting Academy, or the Pedlar’s-French Dict. 112: Ten Shillings Half a Job.