Amr bin Dinar narrated that his father freed his slave during his illness, then he felt that he should take him back (i.e., not free him) and free other slaves instead. So those slaves disputed with me before Abdul Malik bin Marwan, and Abdul Malik validated the freedom of the latter group and annulled the freedom of the former.
Hadith Referenceسنن دارمي / من كتاب الوصايا / 3244
Hadith Takhrijاس اثر کی سند میں دینار مجہول ہیں اس لئے ضعیف ہے۔ «وانفرد به الدارمي» ۔
Brief Explanation
(Commentary on Ahadith 3241 to 3244)
Although the chain of narration of this athar is weak, the correct ruling is precisely this: a person, during his lifetime, may alter any bequest (wasiyyah), and whichever bequest is the last, that alone will be actionable and enforceable.