Praise be to Allah.
Zakaah is due on cash if you have full possession of the nisaab (minimum threshold) and one full hijri year has passed.
The expenses that a father gives to his son is something that the son takes full possession of and disposes of as he wishes, so that is subject to zakaah.
It is well known that cash in the form of bank notes was not known at the time of the Prophet (peace and blessings of Allaah be upon him), but the scholars have ruled that it is subject to zakaah by analogy with gold and silver.
The nisaab for gold is 85 grams, and the nisaab for silver is 595 grams.
If the cash reaches the value of the nisaab for gold or the value of the nisaab for silver, then it has reached the minimum threshold.
As silver is of a lower price nowadays, the nisaab for cash may be worked out on the basis of the nisaab for silver, because that is more on the safe side and is more beneficial to the poor.
Based on the price of silver today – 12 Rabee’ al-Aakhir 1428 AH/29 April 2007 CE – the nisaab for cash is approximately 1093 Saudi Riyals.
If you own this amount of money and one full hijri year has passed since you acquired this amount and it has not dipped below the nisaab during this year, then zakaah must be paid on it, at a rate of 2.5 %.
But if the cash that you possessed dipped below the nisaab during the year, then zakaah does not have to be paid on it, until it reaches the nisaab again and you start to reckon a new year from the time it reached the nisaab.
If the amount by which it dipped below the nisaab was small, then in order to be on the safe side you may pay zakaah, and carry on counting the year. That is because the price of silver fluctuates and does not remain constant throughout the year.
We should not forget to commend your keenness and concern with regard to zakaah, even though you are talking about pocket money that you take from your father, but you are paying attention to Allaah’s right to it and have asked about the Islamic ruling on it at a time when many rich people neglect this pillar of Islam and do not acknowledge Allaah’s rights over their wealth or spend a lot or a little of it, and they spend their days accumulating and hoarding and seeking to acquire more, but at the reckoning on the Day of Resurrection their wealth will be a cause of loss and regret.
Allaah says (interpretation of the meaning):
“And those who hoard up gold and silver (Al‑Kanz: the money, the Zakaah of which has not been paid) and spend them not in the way of Allaah, announce unto them a painful torment.
35. On the Day when that (Al‑Kanz: money, gold and silver, the Zakaah of which has not been paid) will be heated in the fire of Hell and with it will be branded their foreheads, their flanks, and their backs, (and it will be said unto them:) ‘This is the treasure which you hoarded for yourselves. Now taste of what you used to hoard’”
We ask Allaah to bless your wealth for you and to grant you abundant halaal provision.
And Allaah knows best.