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

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.