Musannaf Ibn Abi Shayba 13:556

Abu Bakr narrated to us, he said: Jarir narrated to us, from Mughira, from al-Sha'bi, he said: A man came to Shurayh and said: "I have a runaway slave, and a man is bargaining with me for him. Should I sell him to him?" He said: "Yes. For when you see him (the slave), you have the choice; if you wish, you may ratify the sale, and if you wish, you may not ratify it."