Except with those joined to them in the marriage bond, or (the captives) whom their right hands possess,- for (in their case) they are free from blame,
 The servants they marry are also lawful for them, and this is pointed out specifically so there can be no doubt that former bonded-servants have equal status with people who were always free, lest some people look down upon others. The Prophet said, “There are three types of people who will be given their reward twice: a person from among the Followers of Earlier Revelation who believed in his prophet and then believed in me; a servant who performs his duty well towards God and for whomever he works; and a man who has a maidservant and then educates her, teaches her refined manners, frees her and then marries her.” (Bukhari)
Emerick, Yahiya. The Meaning of the Holy Qur’an in Today’s English (p. 829). Unknown. Kindle Edition.