No shit. Sigh.
As to the question, the supposed ‘correct’ answer is that the slaves could gain experience and skills in {insert something, no idea, underwater basket weaving?} that while yes Massa would have them weaving them baskets underwater, they could accept and keep tips from the people massa farmed them out to work for [in many cases slaves would be owned by Massa Smith living in town, and rented out to Massa Jones who ran a delivery service - so when James made a delivery of a load of lumber, then Massa Doolittle would conceivably tip James for being so polite and delivering quickly. ]
I seriously doubt that normally the slaves saving their tips would have enough to buy themselves free though there was a class of ‘free persons of color’ in New Orleans [digging back into distant memory from living there for a short while] that came about in the 1700s and diminishing in the 1800s from slaves that did manage to buy themselves free, slaves freed through the death of their owners and given themselves in the will [I have no idea how else to describe it] and the assorted partial coloreds [mulatto, octaroon, quadroon, there are a number of various descriptives based upon how much white to black there was ] and also children of white slave owners and black slaves that were freed. It is an unholy tangle.
The house slaves were also given things like older worn or damaged clothing they could remove trim and embroidery from and resell, women would have skills at needlework and be rented out to tailor shops, seamstress shops, hatmakers, or tipped by visitors to the house.
So in theory a slave could ‘better’ themself by learning a valuable skill. Psh, what crap.