Sa'id bin al-Musayyab (may Allah have mercy on him) says that thirty of the Companions of the Messenger of Allah (peace and blessings be upon him) (may Allah be pleased with them) used to say: If a man frees a slave who is jointly owned by him and another man, he (the one who freed) is liable, if he is wealthy. If he is in hardship, then the slave is to be made to work for the remaining value, but without being overburdened.
This too is weak; al-Hajjaj bin Artah is not used as proof. It has also been narrated from al-Hajjaj bin Artah, from Nafi', from Ibn Umar, regarding the slave's working, and that is utterly munkar (rejected).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21385