Henry Louis Gates did a good write up with links to the relevant articles of Blacks who owned slaves here

Basically it seems most Black people who owned slaves owned their relatives, or purchased a slave to ease that slave's burden. Some of these owners did require some work or some sort of payment from the slave. There are also a few instances of Black people owning their relatives and then selling their relative off for some reason. There were also a few Black slave owners who owned many slaves and ran farms/plantations with that labor, especially in Louisiana. It seems that these owners had expectations of their slaves not too dissimilar to white owners.