Sa'id bin al-Musayyib (may Allah have mercy on him) says that a slave-girl belonging to an Arab ran away, went from Wadi al-Qura and joined the very tribe from which she had run away. A man of Banu Udhrah married her, and she became pregnant. Then her master found out about her and took away the slave-girl and her child. Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) ruled that the child belonged to that man of Udhrah, and imposed upon him one slave in exchange for each one; for townspeople he fixed the value of a slave at sixty dinars or seven hundred dirhams, and for a Bedouin six fara'id (camels).
The Shaykh says: This concerns intercourse under a mistaken assumption (of lawfulness), because the child will be free, and he is liable to pay the value to the owner of the slave-girl; and it seems that Sayyiduna Umar (may Allah be pleased with him) held the value to be what is transmitted in this report, if it is established, and Allah knows best. But slavery was applied to Banu al-Mustaliq and Hawazin, which is authentic and established, but later, as a favour, they were freed from slavery out of generosity.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18071