Ibn al-Tayyah came to Sayyiduna Ali (may Allah be pleased with him) and said: I want to make a contract of manumission (mukatabah). He asked: Do you have anything? He said: No. So Sayyiduna Ali (may Allah be pleased with him) gathered the people and said: Help your brother. So they collected wealth for him. He says: Something was left over beyond (what was due on) his contract, so he came to Sayyiduna Ali (may Allah be pleased with him) and asked him about the surplus. Sayyiduna Ali (may Allah be pleased with him) said: Put it towards (other) mukatabs. This indicates that the mukatab is given from the sadaqat, from the share for (freeing) slaves, only what stands between him and his freedom.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21620