If the authorities calculate zakaah as less than it should be
Question

                If what is owed by a farmer is more than what was calculated for him by the committee for collecting zakaah, does the farmer have to give zakaah on the extra amount?.
Praise be to Allah.
    It is proven that the Prophet  (peace and blessings of 
    Allaah be upon him) said concerning zakaah on produce of the earth: “For 
    that which is watered by the sky or a spring, or by its roots, (the rate of 
    zakaah) is one-tenth, and for that which is watered by a camel, (the rate of 
    zakaah) is half of one-tenth.”  

    The Muslim must give this amount of zakaah on that which is 
    produced by the earth, if it reached the nisaab (minimum threshold). 

    If it so happens that those who are in charge of zakaah – 
    namely the committee that estimates the crops and takes zakaah from them – 
    underestimated it, then the owner must give zakaah on the additional amount 
    of crops, whether this extra amount reaches the nisaab or not. That is 
    because the Prophet  (peace and blessings of Allaah be upon him) 
    enjoined a specific share as stated above, one tenth or half of one tenth, 
    so this must be paid. 

    Majmoo’ Fataawa Ibn ‘Uthaymeen 
    (18/64). 

    And Allaah knows best.