He gave him zakaah when he was not entitled to it
Question

                My brother has a mental disability and some people gave us zakaah for him. Please note that my brother is not in need and he has his own income which reaches the nisaab, and zakaah is paid on it. What should we do with this money that came to us? Do we have the right to dispose of it and give it to someone who is entitled to it?.
Praise be to Allah.
      If your brother is independent of means or has someone who 
      spends on him, then he is not entitled to zakaah and it is not permissible 
      for you to accept any money that comes that you know is zakaah. You have 
      to tell the one who is giving zakaah that your brother is not entitled to 
      it, because the Prophet (blessings and peace of Allah be upon him) said 
      concerning zakaah: “No one who is independent of means or who is strong 
      and able to earn a living should have any share in it.” Narrated by Abu 
      Dawood, 1391 and al-Nasaa’i, 2551; classed as saheeh by al-Albaani in 
      Saheeh Sunan Abi Dawood. 

    And he (blessings and peace of Allah be upon him) said: 
    “Charity is not permissible for a rich person, or for one who is strong and 
    healthy.” Narrated by Abu Dawood, 1329; al-Tirmidhi, 589; al-Nasaa’i, 2550; 
    Ibn Maajah, 1829. Classed as saheeh by al-Albaani in Saheeh Sunan al-Nasaa’i. 

    Whatever you have of this money must be given back to the one 
    who gave it, so that he can give it to someone who is entitled to it. But if 
    he delegates you to give it to people who are entitled to zakaah, there is 
    nothing wrong with that. 

    It says in Mataalib Ooli al-Nuha (2/259): If zakaah is 
    given to someone who is not entitled to it, because the giver was unaware of 
    that, the one who took it has to return it to him with any increase in it, 
    whether in weight or in the form of offspring [as in the case of a donated 
    sheep, camel, etc], because it is in increase in the giver’s wealth. End 
    quote. 

    And Allah knows best.