Sayyiduna Sa'id bin al-Musayyib (may Allah have mercy on him) said: "And (also forbidden are) married women" [Surah al-Nisa: 24] refers to women who have husbands, and this goes back to the fact that Allah has forbidden zina.
Imam al-Shafi'i (may Allah have mercy on him) argued that slave-women who have husbands are also unlawful to anyone other than their husbands, and that the exception "except those whom your right hands possess" [Surah al-Nisa: 24] is restricted to female captives only; because when a slave-woman who is not a captive is sold or set free, her sale is not her divorce, since the Prophet (peace and blessings be upon him) gave Sayyidah Barirah (may Allah be pleased with her), when she was set free, the choice of staying with her husband or separating from him. Barirah's ownership ended through her being sold and then set free, so its ending happened in two ways, yet that was not a separation (from her husband). He said: So if the private part of a married woman does not become lawful through the ending of ownership, then when she has not been sold, she does not become lawful through right-hand possession either, until her husband divorces her. He said in his earlier view: Among those who held this view were Sayyiduna Umar bin al-Khattab, Uthman bin Affan, Ali bin Abi Talib, Abd al-Rahman bin Awf and Ibn Umar (may Allah be pleased with them), who said that the husband's marriage remains in place even after she is sold.
And those who say that selling a slave-woman is her divorce are Sayyiduna Abdullah bin Mas'ud, Ubayy bin Ka'b, Imran bin Husayn, Jabir bin Abdullah, Ibn Abbas and Anas bin Malik (may Allah be pleased with them).
The Shaykh said: It seems they drew an analogy with the captive woman, and the hadith of Barirah prevents this analogy. Moreover, there is consensus that whoever marries off his slave-woman does not have the right to have intercourse with her, even though she is among what his right hand possesses; and this is the meaning of the statement of al-Shafi'i (may Allah have mercy on him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13958