All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
The father's speech with his son as regards setting his intention to raise the latter's percentage is considered a promise of a donation but not a fulfilled one, because a legal ruling isn't abiding on the basis of intention alone; thus, a donation won't be abiding save after being actually received as been confirmed by the majority of Muslims' jurists and in accordance to the following narration as follows:" I have gifted to Najsh suit of clothes and containers made out of musk, but I foretold that he will die and my gift will be sent back to me, and if so, then they are all yours." [Related by Ahmad in his Mosnad vol.45 pp.246].
A'isha, the wife of the Prophet, may Allah bless him and grant him peace, said, "Abu Bakr as-Siddiq gave me palm trees whose produce was twenty awsuq from his property at al-Ghaba. When he was dying, he said, 'By Allah, little daughter, there is no one I would prefer to be wealthy after I die than you. There is no one it is more difficult for me to see poor after I die than you. I gave you palm-trees whose produce is twenty awsuq. Had you cut them and taken possession of them, they would have been yours, but today they are the property of the heirs, and they are your two brothers and your two sisters, so divide it according to the Book of Allah." [Related by Malik in his Moattaa' ].
In the aforementioned narrations provide a clear evidence that a donation isn't abiding save after being taken and there is no evidence on transportion of the property because it wasn't recieved in the first place. In this regard, it is stated:" The donation isn't abiding save after being received." [Asnaa' Al-Matalib vol.4 pp. 391].
Consequently, if the father died before writing down and getting the intended raise by his son, then heirs aren't obliged to abide the donation and the inheritance must be divided among them according to Sharia law.
In addition, considering this part of the legacy isn't valid as well, as we answered a question on our site entitled: [A legacy to an heir is not allowable unless the other heirs are agreeable.]. Kindly refer to Fatwa No. (
574). And Allah knows best.