Imam al-Shafi'i (may Allah have mercy on him) said: "This is also narrated from Sayyiduna Umar and Sayyiduna Ali (may Allah be pleased with them both)."
Al-Shaykh (may Allah have mercy on him) said: "Abdullah bin Ahmad bin Hanbal (may Allah have mercy on him) narrated it in Kitab al-'Ilal from Abu al-Rabi' al-Zahrani, from Hushaym, from Sa'id bin Abi Arubah, from Matar, from al-Hasan, from al-Ahnaf bin Qays, from Sayyiduna Umar and Sayyiduna Ali (may Allah be pleased with them both), concerning a free man who kills a slave; they both said: 'His price (is to be paid), however much it may be.'"
And this chain is sahih (authentic).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / Q15956