All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon Prophet Mohammad and upon all his family and companions.

Zakat is one of the pillars of Islam and an obligatory financial act of worship imposed on the wealth of the rich. It must be paid immediately once it becomes due, as it is a right of the poor, and delaying the payment of rightful dues without a valid excuse is prohibited. If one delays it without a valid excuse, they are sinful and remain liable for it. Imam An-Nawawi (may Allah have mercy on him) said: "According to us, Zakat must be paid immediately. If it becomes due and one is able to pay it, it is not permissible to delay it. If one is unable to pay, they may delay it until they are able. However, if they delay it after having the ability, they are sinful and remain liable." [Al-Majmu' Sharih Al-Muhadhdhab/ Vol.5/P.333].

Money that has been separated and set aside as Zakat is not subject to Zakat again because it has effectively become the property of the poor. However, the person liable for Zakat is sinful for delaying its payment if they were able to give it but did not. Imam Abu Is-haq Al-Shirazi (may Allah have mercy on him) said: "If we say that Zakat is due on the wealth itself, and a person possesses the minimum Nisab (threshold) upon which Zakat is obligatory, but does not pay it until another Zakat year passes, then Zakat is not required for the second year. This is because the poor have already become entitled to a portion of that wealth equal to the amount of Zakat due, and as a result, what remains is below the Nisab threshold." [Al-Muhadhdhab/Vol.1/P.268].

In conclusion, the money that is due for Zakat should not be subject to Zakat again. The amount of Zakat due should be set aside each year and deducted from the total wealth in the following year. And Allah The Almighty Knows Best.