Is it permissible for my husband to deduct the excess zakat he paid in the past three years from the current zakat amount?

If one pays more than the obligatory amount for Zakat, this excess is considered a charity for which they will be rewarded, whether it was intentional or by mistake, based on the Almighty's saying: (And whoever volunteers good – then indeed, Allah is Appreciative and Knowing).

It is not permissible to count this excess towards the Zakat of future years, because a condition for accelerated Zakat payment is the presence of intention at the time of disbursement. Al-Shafi'i emphasized this, stating that if one intends a specific amount of their wealth for the Zakat of a particular thing, it is not permissible for them to change that intention to something else after payment.

Furthermore, it is stated in the fatwas of the Permanent Committee and Ibn Uthaymeen that counting the excess towards the Zakat of the following year is not valid due to the absence of intention at the time of payment, and this excess is considered a voluntary charity.