It is essential to adhere to the method specified by the donor in distributing charity
Question

                There is some charity that has been sent to someone to distribute it to some people in a specific manner (one family should take 200 pounds and another family should not take anything). This person thinks that it should be distributed differently so that it may be given to the greatest number of needy people. Is it permissible for him to dispose of it as he sees fit? Please note that the one who sent the charity does not know who is most deserving of it, and he refuses to let it be distributed in any other way.
Praise be to Allah.The person 
    who has been appointed to distribute the charity must adhere to the method 
    specified by the donor, because the deputy is entrusted with doing the job 
    that he is meant to do, and he is the deputy of the one who appointed him. 
    He may only dispose of that for which he has been given permission. If this 
    person thinks that another way of distributing it will be more beneficial, 
    he may explain that to the donor but if he insists on his position then he 
    must adhere to what the giver of the money has stipulated and chosen.  
The Standing 
    Committee for Issuing Fatwas was asked: During Ramadaan, a good person gave 
    me some money and said: Distribute it to the poor because you know more 
    about them than me. I distributed some of it to them, but with the rest I 
    bought some food in the market and distributed it to them instead of giving 
    them the money, because they were unable to buy it from the marketplace 
    themselves, and also because there may be someone in the family who would 
    take the money without the approval of his needy family. Do I have the right 
    to handle the money in this way or not?  
They 
    replied: What you must do is do what the one who appointed you told you to 
    do, by distributing the money to the poor and not buying anything for them 
    with it, because you were not asked to do it in that way. You should pay 
    back what you disposed of and give it to the poor, in accordance with the 
    donor’s instructions and so that you will have discharged your duty, and in 
    sha Allaah you will have the reward for what you spent. 
End quote 
    from Fataawa al-Lajnah al-Daa’imah (14/272). 
This 
    indicates that you should limit yourself to the method of distribution 
    stipulated by the donor. 
And Allaah 
    knows best.