Does zakat apply to money saved from the monthly allowance sent by a father, and what is the nisab and the amount of zakat due on it, and how can the condition of the passing of a year (hawl) and reaching the nisab be reconciled with fluctuating funds?

What you receive from your father is a gift, and you are permitted to dispose of it as you wish in lawful ways. If you save from it an amount that reaches the nisab (minimum threshold) and a full lunar year (hawl) passes over it without it decreasing below the nisab, then zakat becomes obligatory on it. If the money decreases during the hawl, its hawl is interrupted, and you start a new hawl from the day it reaches the nisab again. The nisab for currencies is the equivalent value of eighty-five grams of gold, or five hundred and ninety-five grams of silver. The amount that must be given is two and a half percent (a quarter of a tenth).