Sa'id bin al-Musayyib (may Allah have mercy on him) says that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) fixed a ransom for every slave: for an Arab slave, six camels. He gave this ruling regarding those who had married Arab slave-girls.
This narration too is mursal, but it is good (jayyid).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18070