Abu Yahya al-A'raj (may Allah have mercy on him) says that the Prophet (peace and blessings be upon him) was asked about a slave whose master freed him at the time of his death, having no other wealth and being in debt. The Prophet (peace and blessings be upon him) said: "He is to be made to work to pay off the debt."
This is munqati' (its chain is broken), and its narrator is al-Hajjaj bin Artah, who is not used as proof.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21384