He owned a slave named Fennesz, who was tasked with educating the children of the household. After Daenerys Targaryen took the city and freed all the slaves, Fennesz asked her to be allowed to re-sell himself into slavery, as Mighdal and his family had treated him well. The queen allowed for Fennesz and Mighdal to sign a contract of employment, but only for a period of one year at a time.
In the books
The characters of Mighdal and Fennesz were invented to present a problem alluded to in the books, the plight of slaves whose lives actually worsened with freedom.