A spiff or spiv is an immediate bonus for a sale. Typically, "spiffs" are paid, either by a manufacturer or employer, directly to a salesperson for selling a specific product.
An early reference to a spiff can be found in a slang dictionary of 1859; "The percentage allowed by drapers to their young men when they effect sale of old fashioned or undesirable stock." An article in the Pall Mall Gazette of 1890 on the practices in London shops uses the term:
... a "spiff" system is usually adopted, spiffs being premiums placed on certain articles, not of the last fashion, indicated by a marvellous hieroglyphic put on the price ticket. These marks are well known by the assistant, and the almost invisible mystic sign explains why an article, wholly unsuitable, is foisted on the jaded customer as "just the thing."
It seems to be connected with the use of the word in that period to mean a dandy or somebody smartly dressed (hence spiffy, and to spiff up - to improve the appearance of a place or a person), but nobody seems to have been able to disentangle the threads of which came first, or what influenced what, or where the word originally came from.
In 1947 it was reported that spiffs were prizes given to employees who sold particularly high amounts of electrical goods.
Further reading 
- Andris A. Zoltners, Prabhakant Sinha, Sally E. Lorimer (2008) The Complete Guide to Sales Force Incentive Compensation, Amacom ISBN 0814473245
- F. Caldierero & A. T. Coughlan (2007) Marketing Science 26 (1) pp 31-51 "Spiffed-up channels: The role of spiffs in hierarchical selling organizations"
- Oxford English Dictionary (1989) 2nd edition
- Pall Mall Gazette (London, England), Wednesday, April 2, 1890, The Ladies Corner: Round the Clock with a London Draper
- Daily Mirror, 11 Jan 1963, p 16 "Where the spivs began"
- TIME magazine Nov 17, 1947 "Retail Sales: Spiff Spiked"