(1) If you have given him after you considered him eligible for zakah then your zakah was paid and the person shall be sinful who has deceived. But if you were already doubtful whether he was eligible for zakah or not or you were not sure about him then in such case it is not lawful to give zakah. Zakah shall not be paid till you are sure that the person is eligible for zakah.
Allah knows Best!
Darul Uloom Deoband