[Abu 'Eisa said:] This Hadith is Hasan Gharib. And this acted upon according to most of the people of knowledge among the Companions of the Prophet (ﷺ) and others: The Mukatab is a slave as long as something remains due from him for his Kitabah.
Al-Hajjaj bin Artat reported similarly from 'Amr bin Shu'aib.
In-book : Book 14, Hadith 62
English translation : Vol. 1, Book 12, Hadith 1260