Am I missing something here? "Enslaved" people were paid after all, and required small change for their purchases?

You're absolutely right, of course, that the experience of slavery over a time and varying places is quite different. In the Caribbean in the 18th century, on many of the islands, enslaved people had provision grounds where they were expected, when not working on the plantation, to grow their own food and sometimes raise small livestock to feed themselves. When and if they had surpluses, these were sold at weekly markets -- and white and black inhabitants alike relied on the markets for much of their food.

Overseers and planters would also occasionally give certain slaves small gifts of money for specific tasks or as some form of reward. These are all small transactions -- thus the need for small change. But enslaved people are not being paid for their daily labor -- this is just small amounts of money they earn from growing vegetables and raising chickens in their free time, on top of everything else they do.