Ibn Awn narrates from Muhammad that a mukatab said to his master: Collect your contract payment from me. He said: No, only in fixed installments. He came to Sayyiduna Uthman (may Allah be pleased with him) and mentioned this to him, so he summoned the master and said: Take your contract payment. He refused, saying: I will take only the fixed installments. Sayyiduna Uthman (may Allah be pleased with him) had the money brought and placed it in the public treasury, freed him, and said: I will pay you your portion. When he saw this, he then took the money.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21710