Asalam u alaikum
My wife recently sold her gold ornaments when We came to know that they were of a lower carat and as a result of a lower value than we had assumed earlier. As a result it came to the fore that the zakat we have paid on the gold for the last two years is substantially more than what was due because we had overvalued its price due to mistaken assumption of it being of a higher carat. Now the zakat of current year is due and my question is can we adjust the overpayment of zakat in the last two years in this years zakat?
Last year we paid 40000 but we actually should have paid 25000 only.
This year the zakat due is 25000. Can we adjust the 15000 overpayment of last year and pay only 10000 this year

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The extra amount beyond the due Zakaah (payable by your wife) is considered as optional charity. Accordingly, it is not sufficient to deduct it from the Zakaah for the current year (in terms of clearing her liability from the obligation in this regard) given the absence of the intention (when she spent the extra amount), which is a condition for the validity of the Zakaah payment. Shaykh Ibn ‘Uthaymeen was asked a similar question cited in Liqaa’ Al-Baab Al-Maftooh. It reads: “Question: Your Eminence, a man owns a real-estate property and wanted to pay Zakaah on it. He paid 100 riyals for every 1,000 riyals (10%) and was informed afterward that he should have paid 25 riyals only for every 1,000 riyals (2.5%). What is the ruling in this regard?

Answer: The extra amount beyond the due Zakaah payment is considered charity. If a person pays an extra amount of Zakaah believing that it is the obligatory amount he has to pay, then the extra amount is regarded as charity. Allah, The Exalted, may grant him equal rewards to those for Zakaah, which are greater than the rewards for an optional charity. This man paid 10% instead of 2.5%; the extra amount is 7.5%. Had he paid such amount in charity, he would have earned the rewards for charity. However, if he pays them as Zakaah, he earns greater rewards because Allah, The Exalted, Said in the Qudsi Hadeeth: ‘My slave draws not near to Me with anything more loved by Me than the religious duties I have enjoined upon him.’ However, one may ask, ‘If this man intended to consider that extra amount (7.5%) to be (deducted from) the Zakaah payments of three coming years, would this be valid in terms of clearing his liability?’ No, this would not be valid in terms of clearing his liability, because the Prophet said: ‘Indeed, actions are judged according to the intentions, and every person will get the reward of what he has intended.’” [End of quote]

It should be noted that the jewelry intended for lawful use (as adornment) is not subject to Zakaah according to the majority of the scholars. However, paying Zakaah on such jewelry is more prudent in order to remain on the safe side in terms of clearing one’s liability and to avoid the difference of opinion, as some scholars held that it is obligatory to pay Zakaah on such jewelry.

Allah Knows best.