In Huck Finn, there's a moment where Jim talks about investing in "stock" by which he means livestock. He also mentions lending money to another slave who was starting a bank.
Obviously that's mostly Mr. T having a bit of fun, but would a slave have had extra spending money in the 1860s South? What kinds of things would they spend it on?
This older answer discusses the peculium (money earned/held by an enslaved person) in the American system of slavery, so should be of interest.