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).
Amr bin Shu'ayb narrated from his father, from his grandfather, that Sayyiduna Umar (may Allah be pleased with him) said: If a free man kills a slave, then what is due for him is his price.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15956