Praise be to Allaah.
If a person pays zakaah more than the amount that is
required of him, thee extra amount is regarded as charity (sadaqah) for
which he will be rewarded, because Allah says (interpretation of the
meaning): “And whoever does good voluntarily, then verily, Allaah is
All-Recogniser, All-Knower” [al-Baqarah 2:158].
It is the same whether this additional amount was given
deliberately or by mistake.
But he cannot count this additional amount as zakaah for the
coming years, because one of the conditions of giving zakaah in advance is
that it should be intended as such at the time of giving it.
Imam al-Shaafa‘i mentioned an issue that is similar to this:
a man paid the zakaah on his wealth before the end of the year, then his
wealth was destroyed before zakaah became due on it, and he wanted to
allocate the wealth he had paid as zakaah on other wealth that he had: is
that acceptable?
Imam al-Shaafa‘i said: He does not have the right to do that,
because his intention was to pay it as zakaah on specific wealth that he
had. So he does not have the right to divert his intention concerning it
after paying the money to those who were entitled to it.
End quote from al-Umm, 2/24
Shaykh Muhammad ibn ‘Uthaymeen (may Allah have mercy on him)
was asked: I paid twenty thousand riyals as zakaah for the year 1411 AH,
then I calculated the zakaah on my wealth for the same year, and I found
that it was fifteen thousand. Is it permissible to regard the extra amount
as zakaah for the year 1412 AH, even if that is without intention (niyyah)?
He replied:
This questioner paid more zakaah than was required of him,
and he is asking whether he can count it towards the zakaah of the next
year.
I say that he cannot count it towards the zakaah of the next
year, because he did not intend it as such. Rather it is charity (sadaqah)
which will bring him closer to Allah, may He be glorified and exalted,
because the Prophet (blessings and peace of Allah be upon him) said:
“Actions are but by intentions, and each person will have but that which he
intended.”
End quote. Majmoo‘ Fataawa al-Shaykh Ibn ‘Uthaymeen,
18/308
And Allah knows best.