Is zakat obligatory on a sum of money that a father left for his daughter after his death to complete her marriage dowry, knowing that she is not yet married and has not included it among her previous assets?

The money is not your private property as long as you returned it to your father. For a gift to be valid, possession (al-hawz) is a condition, and returning it invalidates it. The division of wealth is obligatory among the heirs according to their prescribed shares, unless they waive their rights to it in your favor. In that case, it becomes your property, and Zakat becomes due on it after one year (hawl) from the time of their waiver, provided it reaches the niṣāb. If they do not waive their rights, each heir must pay Zakat on their share upon receiving it for all past years, provided it reaches the niṣāb.