Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) said: "Whoever frees his share in a slave, it is his responsibility to free him completely if he has wealth; and if he has no wealth, then only as much will be freed as he freed."
(b) In the hadith of Abu Bakr and Uthman it says: "It is his responsibility to free him completely if he has wealth; if he has no wealth, a fair value shall be assessed, that is, on the part of the one who freed, as much has become free as he freed."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21361