Musannaf Ibn Abi Shayba 13:1652

Abu Bakr narrated to us, he said: Asbat bin Muhammad narrated to us, from Mutarrif, from al-Hakam, regarding a slave who came to a man and gave him wealth and said, "Buy me." So he bought him and then emancipated him. Then [the master] became aware of that. He said: The sale is permissible, and the price with which he bought the slave is to be taken [from the slave].