lagniappe
An extra or unexpected gift or benefit, such as that given to customers when they purchase something.
Lefe had been successful, and was supposed to have amassed quite a "pile," which he was very loth indeed to part with; and when he lost, if the money were not absolutely staked, would usually put off the winner with some old horse that he had fixed up for sale, or a dubious note that he had received as "lanyappe," (Anglice, boot money.)
Where it comes from
Borrowed from Louisiana French lagniappe, from Spanish la ñapa, a variant of yapa (“small gift or additional quantity given to a valued customer”), from Quechua yapa (“addition, increase, supplement; lagniappe”), yapay (“to add, to increase”).
Adapted from the Wiktionary entry lagniappe (CC BY-SA 4.0).