Praise be to Allah the Lord of the Worlds.
If a person pays the Zakah of his wealth to someone and finds out that the latter isn`t among the eligible recipients of Zakah, then it doesn`t avail him (Giver) and is obliged to repay that Zakah. Moreover, it is permissible for the giver to reclaim that Zakah from the ineligible recipient if he had told the latter, upon handing him the money, that it is the Zakah of his (Giver) wealth.
An-Nawawi (May Allah have mercy on him) said: "If he pays theZakah of his wealth to someone thinking that he is an eligible recipient, but he turned out to be a slave, a disbeliever, a Hashemite or of Banu Muttalib, then he is required to take it back. If he succeeds in that then he should give it to another poor Muslim. If he doesn`t, then- according to the Shafie doctrine-the given Zakah doesn`t avail him (giver) and he is required to repay it to an eligible recipient."{Al-Majmou`, vol. 6 :pp.231}.
However, Maliki jurists and one opinion of the Shafies permit paying Zakah of wealth to the poor and needy from Al-al-Bayt{Family of the Prophet Mohammad (PBUH) if they are denied the one-fifth they are entitled to from the war booty. This is in order to spare them begging others. Kindly refer to [Al-Shareh Al-Kabeer, vol.2:pp.212] by the Maliki Sheikh Al-Dardeer.
An-Nawawi (May Allah have mercy on him) said: "If Banu Hashim and Banu Al-Mttalib were denied the one-fifth they are entitled to receive from the war booty, are they allowed to take from the Zakah money? There are two opinions; the most correct is that this isn`t permissible while the second states that it is permissible…………….." {Al-Majmou`, vol.6:pp.220}.
In conclusion, Zakah avails the person who imitates the Maliki doctrine and some Shafie jurists as regards giving the Zakah of wealth to descendants of Prophet Mohammad`s family. However, the reliable opinion of our doctrine (Shafie) is that giving Zakah to a Hashemite doesn`t avail the giver at all. And Allah the Almighty Knows Best.