It is narrated from al-Hasan (may Allah have mercy on him) that when a man makes a bequest, he may change his bequest whenever he wishes. Al-Hasan (may Allah have mercy on him) was asked: What about a freed slave? Al-Hasan (may Allah have mercy on him) said: Whether a slave or anything else.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الوصايا / 12655