Green’s Dictionary of Slang

spiffs n.

[note drapery jargon spiff, a percentage allowed to salesmen when they sell off old or unfashionable stock]

(US Und.) bonus money, extras.

[[UK]Hotten Dict. of Modern Sl. etc. 98: spiffs the percentage allowed by drapers to their young men when they effect sale of old fashioned or undesirable stock].
[Aus]‘The Wayback Family’ in Sun. Times (Sydney) 16 Dec. 5/1: The ‘nice’ young man rubbed his hands secretly at the many ‘spiffs’ he was scoring for himself by working off all the old goods in stock.
[US]T. Minehan Boy and Girl Tramps of America (1976) 84: When I see a woman going home I follow her and hit her for some clothes and lots of times I get a big dinner, and some spiffs too.
[US]Monteleone Criminal Sl. (rev. edn).