I can't speak specifically to the film, as I have not seen it, but I can answer about slaves independent production and wage earnings.

First, in regard to his playing an instrument. There is evidence that in the United States that a number of enslaved persons were able to play musical instruments, and they had a particular affinity for the fiddle. Between 1745 and 1800, there are about 837 runaway slaves advertised in the Maryland Gazette. Of these, at least 30 knew how to play the fiddle or banjo; Philip Morgan found 50 runaways in Virginia in the eighteenth century (Database in possession of the author; Morgan, Slave Counterpoint, pp. 591-593). So the idea that a slave could play a violin outside his working hours is completely plausible.

Next, many slaves were able to earn an income doing work outside their plantation, or away from their master. I would highly recommend checking out the essays in Morgan & Bailyn, ed. The Slaves' Economy: Independent Production by Slaves in the Americas, which presents several studies of slaves earning their own income. One of the most important ways slaves did this was by growing their own provision grounds. This was more particular to the Caribbean, but can apply to North America. Slaves were usually granted a day off from work on the plantation to tend to their own "garden plots" where they would grow their own food crops to supplement their provided rations. Slaves grew corn, wheat, potatoes, manioc (cassava), or even cash crops like tobacco. The slaves could then either consume their own crops, or sell the excess to either their master, to another slave, or in a an open market (the last of these being most common in the Caribbean).

Slaves could also practice their trade outside their master's estate. Occasionally a slave could keep the full income they would make to themselves, but their master could claim a portion. The most common occupations for a slave would be a carpenter, cooper, blacksmith, barber, and shoe maker. Hiring out slaves was common practice, especially in the Chesapeake, whereby masters would allow their slaves to be hired by another person for wages, paid mostly to the master (Morgan, Slave Counterpoint; Rockman, Scraping By).

Lastly, you ask what would slaves spend their money on. There are a lot of possibilities, but the two items we have the most evidence for are food/drink, and clothing. Slaves frequently had a standard weekly allotment of food consisting of some meat, flour or corn, and perhaps water, beer, or liquor. In order to expand on this diet, slaves would grow or purchase vegetables like squash, potatoes, or cabbage. They would also want to purchase salt, pepper, or whatever spices could be available. Besides foods to supplement their diets, slaves often bought liquor with their earnings. Slaves' owners would often give their slaves a few ounces of liquor at special occasions, but based on the newspapers advertisements we see for slaves, liquor was a hot ticket item. Somewhere around 25% of the advertisements for runaways in the Maryland Gazette make reference to liquor.

The last item would be special items of clothing. There was a standard osnaburg or wool clothes that were provided to the slaves from their masters. However, slaves sought to decorate themselves with accessories or unique items in the same way that a free person might. As just one example, a slaves named Harry ran away in 1779 wearing "a blue coat with a red cape [collar], made regimental fashion," (Maryland Journal July 6, 1779). Harry may have stitched the pieces together himself, as the colors would have been striking against the beige/brown of most slaves clothing.