Hmam bin Yahya bin Dinar همام بن يحيى بن دينار · The Book of Manumission of Slaves

on his father's authority, told that his grandfather reported the Prophet (ﷺ) said: A slave who has entered into an agreement to purchase his freedom is a slave as long as a dirham of the agreed price remains to be paid.

Safinah said: I was a slave of Umm Salamah, and she said: I shall emancipate you, but I stipulate that you must serve the Messenger of Allah (ﷺ) as long as you live. I said: Even if you do not make a stipulation, I shall not leave the Messenger of Allah (ﷺ). She then emancipated me and made the stipulation with me.

On his father's authority (this is AbulWalid's version): A man emancipated a share in a slave and the matter was mentioned to the Prophet (ﷺ). He said: Allah has no partner. Ibn Kathir added in his version: The Prophet (ﷺ) allowed his emancipation.