Musannaf Ibn Abi Shayba 18:488

Abu Bakr narrated to us, he said: Abd al-A’la narrated to us, from Dawud, from al-Sha’bi, that Sa’id ibn al-’As set the blood money for a slave who was killed by mistake at four thousand, and his price was more than that. And he said: "I dislike setting his blood money at more than the blood money of a free man."