Does Zakat become obligatory on the current amount when Zakat is remembered, or on the amount that existed on the date Zakat became due?

Zakat on wealth must be disbursed upon the completion of its year (hawl) and the attainment of the a mount (nisab). It is not permissible to delay it beyond its due time unless there is a legitimate شرعي excuse. A person who forgets is not sinful, but must hasten to disburse it.