Another example of slaves earning money through their own industry (outside
their regularly assigned tasks) may be found in Harriet Jacobs' Incidents in
the Life a Slave Girl.  Her grandmother put aside a good deal of money made by
selling good crackers she made.  Of course, the book also records that her
grandmother lent the money to her mistress who never paid her back--indicating
the precarious state of "property" owning property...

Cheers -- Kevin