Is the amount of money given by the mother to her eldest daughter to hand over to her youngest daughter upon her marriage considered a will that requires the permission of the heirs, and is it permissible to give it to the youngest daughter before her marriage, and is zakat obligatory on it, and what should the eldest daughter do with the money if the youngest daughter does not get married?

This gift is considered a bequest, and it is not permissible for an heir unless the other heirs approve it. The bequeathed money is included in the estate and divided according to Sharia, unless the heirs approve it. If they approve it, the money becomes the property of the legatee from the time of approval. Therefore, Zakat on the money becomes due after one year from the date of death if the bequest was not approved, or from the date of the heirs' approval and acceptance if it was approved. However, if the gift was given during the giver's lifetime but not delivered to the recipient before the giver's death, it reverts to the estate.