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