Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) was asked about a man who makes a bequest and the heirs willingly give their permission, then (later) go back on it, and he said: "That is a reprehensible act; it is not permissible."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9130