All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
It is permissible for you to take a fee from your relative in return for distributing the zakah of his money. However, you must agree with him about this fee because it is not permissible for you to take anything from the money without his knowledge. Also, it is a condition that this payment should not be from the zakah money because you are not among those who are employed to collect it and who are permitted to take from it.
As regards the amount of the fee (payment), then this depends on the agreement between you and whoever entrusts you to distribute his zakah.
Allaah knows best.