Musannaf Ibn Abi Shayba 18:474

Abu Bakr narrated to us, he said: Abd al-Rahim ibn Sulayman narrated to us, from Muhammad ibn Salim, from al-Sha'bi, regarding a slave who killed a free man intentionally. He said: It is for the family of the killed person; if they wish, they may kill him, and if they wish, they may take him as a slave.