He gave his zakaah to a poor man who turned out to be well off

The scholars differed concerning one who gives his zakaah to
one who is well off, thinking that he is poor. The Shaafa’is are of the view
that it does not count as such. See al-Majmoo’ (6/225). 

The Hanbalis are of the view that it does count, and this is
the correct view. 

This is indicated by the report narrated by al-Bukhaari
(1421) and Muslim (1022) from Abu Hurayrah (may Allaah be pleased with him),
that the Messenger of Allaah (peace and blessings of Allaah be upon
him) said: “A man said, ‘I am going to give charity’  … He went out with his
charity and placed it in the hand of a rich man. The next morning, they
said, ‘Last night he gave charity to a rich man.’ He said: ‘O Allaah, to You
be praise for a rich man. … It was said to him: ‘As for your charity, it has
been accepted. … As for the rich man, perhaps he will learn a lesson and
spend from that which Allaah has given him.” 

A person’s poverty may not be apparent, so it is sufficient
for the one who is giving zakaah to think that he most likely poor. Allaah
says (interpretation of the meaning): 

“The one who knows them not, thinks that they are rich
because of their modesty. You may know them by their mark, they do not beg
of people at all”

[al-Baqarah 2:273]. 

Al-Bahooti said in Kashshaaf al-Qinaa’ (2/296): If he gives
him zakaah thinking that he is poor then he turns out to be well off, that
is still valid. End quote. 

Does the same ruling apply in all cases where he gives zakaah
to people thinking that they are entitled to it, then he finds out that they
were not entitled to it, such as one who gives his zakaah to a kaafir,
thinking that he is a Muslim, or he gives it to a poor man, the finds out
that he is one of the descendents of the Prophet (peace and blessings
of Allaah be upon him)? 

The Hanbalis are of the view that it dos not count as zakaah
in these cases, because in most cases such things are not hidden from
people, unlike poverty, which may be hidden. See al-Mughni (2/281). 

Shaykh Ibn ‘Uthaymeen favoured the view that it does count as
zakaah in these cases. He said in al-Sharh al-Mumti’ (6/265): If he
gives it to someone who he thinks is entitled to it after checking on him,
then he finds out that he was not entitled to it, that still counts as
zakaah, because he feared Allaah as much as he could, and Allaah says
(interpretation of the meaning): 

“Allaah burdens not a person beyond his scope”

[al-Baqarah 2:286].  

What counts in acts of worship is what the person thinks,
unlike interactions with others, where what counts is the reality of the
transaction, not what the person thinks.  

End quote.