What is the amount of zakat due on money invested for 24 years, and is it to be paid from the principal or from the investor's salary?

If Zakat (definition) is obligatory on mudarabah (definition) funds, it must be disbursed immediately. Delaying the disbursement of Zakat does not nullify it, no matter how many years pass. Rather, it remains a debt owed by the individual, and one must hasten to disburse it for all past years, whether the wealth belonged to the father or the children. If the owner of the wealth dies and Zakat is due on him, the Zakat is to be disbursed from his wealth before the inheritance is distributed. It is permissible to disburse Zakat from wealth other than that on which Zakat became obligatory, such as disbursing it from one's salary, for example.