All perfect praise be to Allah The Lord of the Worlds. May His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
Zakah (Obligatory charity) is a debt on the deceased and must be paid for the entire period that preceded the division of the estate; provided that it is ascertained that the deceased hadn`t paid it. The Zakah must be calculated from the day his/her total wealth met Nisab (Minimum amount liable for Zakah) and a full lunar year passed over possessing it (Nisab). Accordingly, the Zakah of previous years must be given before the estate of the deceased is divided amongst the heirs.
The money that is separated and kept as Zakah isn`t liable for Zakah again because it has become the property of the poor. In addition, the person who delays paying Zakah due on his/her wealth, while able to do so, incurs sin.
Imam Al-Shirbini (May Allah have mercy on him) said: "Zakah pertains to the wealth or property liable for it; just as the company`s wealth pertains to the partners, as indicated by the evidence stated in Sharia texts. Another view states that Zakah is like a mortgage….. whereas a third view, which is the weakest, states that Zakah pertains to the giver himself and not the property, such as Zakat al-Fitr. " {Moghni Al-Mohtaj Vol.2:P.137}. For more information, kindly check {Al-Mohazab Fi Fiqh Al-Imam Al-Shafie Vol.1:P.268}. And Allah the Almighty knows best.
[1] Zakat al-Fitr, or the Zakat of Breaking the Fast of Ramadan, is the special obligatory alms paid by all Muslims at the end of the Ramadan fasting month.