My thanks to Neil McDonald for his very helpful response to my query.

There is evidence that many of Jefferson's slaves were raising various food
products on the side, on a rather substantial and impressive scale, for sale
to the big house and off the plantation.  Jefferson did make cash payments
to his slaves, which are recorded in his Memorandum Book.  In some instances
he borrowed cash from his slaves.  One day he paid his man Jupiter and then
borrowed money right back.  The soap sale caught my eye because it was
probably not a one-time thing.  Jenny may have been "in the business" of
making soap in quantity for sale or exchange in the slave quarter, and when
Jefferson ran short he bought some.  A number of Jefferson's people received
regular gratuities, and others got a cut of profits from nailmaking,
coopering, charcoal burning, etc. as an incentive.  It's an interesting
contrast with GW's practices at Mount Vernon, where it seems GW never paid a
farthing to a slave for anything.  The exception was his cook Hercules, who
made a very large amount of money in Philadelphia selling the surplus from
GW's dinners (I hesitate to call them left-overs), with GW's approval.  GW
wanted to keep Hercules happy because he was so skilled at his work; but
when he got the chance Hercules ran for freedom.  

Henry Wiencek