Ibn Tawus (may Allah have mercy on him) says: Ibn al-Munkadir (may Allah have mercy on him) asked me: "What did your father use to say: may the owner of a mudabbar slave sell him?" I replied: "He may sell him when he needs his price." Muhammad bin al-Munkadir (may Allah have mercy on him) said: "He may sell him even without need."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المدبر / 21566