Ali and Ibn Abbas (may Allah be pleased with them both) said: "When a free man deliberately kills a slave, it is (a case of) retaliation."
Ali bin Umar (al-Daraqutni) said: No proof is established by it, because it is mursal.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15941