Missed Zakat becomes obligatory after a person qualifies to pay Zakat. This obligation remains upon a person despite negligence, error or ignorance.
The obligation of Zakat according to the Hanafi School is originally upon the wealth but is transferred onto an individual upon exhausting his wealth. Whereas, the other schools of Fiqh opine that the obligation of Zakat is vested on an individual from the outset. Thus, it remains obligatory.
And Allah knows best!
Reviewed on 30/11/2021