Whether you take slavery in non-Islamic Africa or the Islamic world, social mobility was not at all restricted. It was only chattel slavery of the North American capitalist variety that rigidly restricted mobility from slave to non-slave-owning citizen or to slave owner (and vice versa).
<http://www.digitalhistory.uh.edu/database/article_display.cfm?HHID=64> Hereditary slavery, extending over several generations, was rare. Most slaves in Africa were female. Women were preferred because they bore children and because they performed most field labor. Slavery in early sub-Saharan Africa took a variety of forms. While most slaves were field workers, some served in royal courts, where they served as officials, soldiers, servants, and artisans. Under a system known as "pawnship," youths (usually girls) served as collateral for their family's debts. If their parents or kin defaulted on these debts, then these young girls were forced to labor to repay these debts. In many instances, these young women eventually married into their owner's lineage, and their family's debt was cancelled.
Under a system known as "clientage," slaves owed a share of their crop or their labor to an owner or a lineage. Yet they owned the bulk of their crop and were allowed to participate in the society's political activities. These slaves were often treated no differently than other peasant or tenant farmers. -- Yoshie <http://montages.blogspot.com/> <http://mrzine.org> <http://monthlyreview.org/>