I don't know about African slaves, but Chinese girls and young women were bought and sold as prostitutes regularly in San Francisco in the late 19th century.

How many is unknown. More than one, less than six thousand basically. There were reported to be 68 Chinese brothels in SF in 1885, 567 prostitutes and 761 questionable others by a special committee of the city (altho many were in fact houses of independent women called "sworn sisters" not working in prostitution). The question of how many had always been inflated by whites in the City, a Chinese man testified to that committee there were two or three hundred prostitutes, but that estimate was "not taken seriously." There were a "large number" of brothels in 1900 according to a Chinese newspaper.

The most famous prostitute/slave was Xijiao, who worked in a brothel on modern day Ross Alley--adopted at a young age, raised to be a prostitute. After a long time in the profession (unusual as venereal disease usually killed most prostitutes off by their early 20s) at which point she became very well known, she fell in love with a man who paid several hundred dollars for her freedom and later killed her with a knife on December 8, 1875.

Sources:
The Barbary Coast: An Informal History of the San Francisco Underworld

Chinese San Francisco, 1850-1943, ch 3, The Social Landscape

edit: sorry about the edits to clear the numbers up, had to reread the chapter again.