The promise made by the mother to give each of her daughters a gold coin upon graduation is a promise of a gift. The daughters who received their gift upon graduation have a right to their gift. It is their property and is not part of their mother’s estate.
On the other hand, the rest of the gold coins that were not given out during the mother’s lifetime are part of the estate. They constitute part of the property that will be divided out among the heirs according to the shares fixed by Islamic Law.
Abû Bakr had promised to give his daughter `A’ishah a gift of twenty bushels from the produce of one of his farms. However, `A’ishah did not take the fruit until the time of her father’s death approached.
Thereupon, Abû Bakr said to her: “O `A’ishah, I had promised you a gift of twenty bushels. If you have already taken possession of it, then it is yours. If not, then it is now the property of the inheritors.”
However, the heirs might agree of their own free will to waive their rights over the remaining gold coins, so that the mother’s wish and promise can be fulfilled. It is permissible for an heir to waive his or her right, as long as it is done freely without any duress.
In that case, the coins will be removed from the estate and set aside for the girls. Then the remainder of the estate will be divided among the heirs according to the shares fixed for them by Islamic Law, without reckoning in the value of the coins.
And Allah knows best.