Abu al-Hajjaj Mujahid (may Allah have mercy on him) narrated from Sayyiduna Jabir bin Abdullah al-Ansari (may Allah be pleased with them both) that in the mosque of the Messenger of Allah (peace and blessings be upon him) there was a man of the tribe of Banu Udhrah, known as Abu Madhkur. He had a Coptic slave whom he had made a mudabbar. Then he fell into need, and the Messenger of Allah (peace and blessings be upon him) said: "When you yourself are in need, begin with yourself first." Then the Prophet (peace and blessings be upon him) sold him to Nu'aym bin Abdullah (may Allah be pleased with him) (a man of the tribe of Banu Adi bin Ka'b) for eight hundred, and he benefited from it.
Mujahid (may Allah have mercy on him) and the jurists of Makkah held that tadbir is a bequest in which its maker has the choice as long as he lives: he may carry out of it whatever he wishes and revoke of it whatever he wishes.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المدبر / 21557