"Tipping as an American practice stretches back centuries. 'There are records of George Washington and Thomas Jefferson giving tips to their slaves'..."

"... said Michael Lynn, a professor of consumer behavior at Cornell University’s School of Hotel Administration, who has studied changes in tipping habits. In the 1940s, he said, the average restaurant tip was about 10 percent. 'It’s very clear that tip sizes have increased over time,' he said, adding that he could not predict how high they would go."

From a NYT article titled "$3 Tip on a $4 Cup of Coffee? Gratuities Grow, Automatically."

So iPads are being used these days to prompt customers to give huge tips, but I was interested in the history of tipping and the tipping of slaves. I found "The Private Life of George Washington's Slaves":

A slave might receive a tip for special services. In the spring of 1768, when George Washington left the home of his brother-in-law, Burwell Bassett, he was probably acting properly when he left fifteen shillings and nine pence for the "Servants," who would have had extra duties to perform in caring for him or for any other houseguest. When a slave who belonged to overseer James Cleveland returned a horse to Mount Vernon in the summer of 1783, he received three shillings, which he could, presumably, spend as he wished. In 1785 Washington gave several slaves six shillings in gratitude for their assistance in getting his valet, William Lee, to the home of a friend, Dr. David Stuart, after Lee broke his kneecap while surveying with Washington. (It is unclear whether the people were from Mount Vernon or from Stuart's home, or if they just happened upon the accident.) Washington's contemporaries, benefitting from the help of one of his slaves, would quite likely have tipped them as well.

Where someone of Washington's social class paid a slave for service with cash, a person of lower rank might barter something in their possession. In 1784, for example, a young, indigent white woman stole from the wife of Charles MacIver, a Highland Scot living in Alexandria, garments including a dress (of Indian chintz with a white ground and red stripes) and an apron. The thief gave the dress and apron to one of Dr. Stuart's slaves in exchange for ferrying her across a body of water; the slave then gave the clothes to his wife.

That second link has other details about slaves making money, including by selling their teeth:

Since at least the end of the Middle Ages, poor people had often sold their teeth for use in both dentures and in tooth-transplant operations for those wealthy enough to afford the procedures. Sometimes the teeth were perfectly healthy; others were diseased and needed to be pulled anyway....

[I]n May of 1784, Washington paid several unnamed "Negroes," presumably Mount Vernon slaves, 122 shillings for nine teeth, slightly less than one-third the going rate advertised in the papers, "on acct. of the French Dentis [sic} Doctr. Lemay [sic]," almost certainly Le Moyer. Over the next four years, the dentist was a frequent and apparently favorite guest on the plantation. Whether the Mount Vernon slaves sold their teeth to the dentist for any patient who needed them or specifically for George Washington is unknown, although Washington's payment suggests that they were for his own use. Washington probably underwent the transplant procedure--"I confess I have been staggered in my belief in the efficacy of transplantion," he told Richard Varick, his friend and wartime clerk, in 1784--and thus it may well be that some of the human teeth implanted to improve his appearance, or used to manufacture his dentures, came from his own slaves.

Also, the slaves' dogs:

His slaves' ownership of dogs also troubled and economically threatened George Washington. They apparently trained the animals quite well. "It is astonishing to see the command under which their dogs are," Washington commented to his manager Anthony Whiting in 1792. Although the slaves probably kept the dogs ostensibly for hunting, both men felt that they used the dogs during "night robberies" to round up Mount Vernon sheep, which they then sold to certain outside "receivers." Washington and Whiting also feared that dogs might kill the sheep. Washington eventually ordered Whiting to decide which dog or dogs to keep on each farm, then kill all the others. Afterward, "if any negro presumes under any presence whatsoever, to preserve, or bring one into the family. . .," Washington proclaimed, "he shall be severely punished, and the dog hanged." Washington was not the only plantation owner to resort to such drastic measures; Thomas Jefferson, on at least one occasion, ordered the destruction of all dogs belonging to his slaves, while permitting his overseer to retain a pair for his own use. At least one of the condemned dogs was hung as a disciplinary warning to the Monticello slaves.