Sheikh Muhammad b. Sâlih al-`Uthaymîn
You may not consider the extra amount that you have paid to count towards the following year’s Zakâh.
The Prophet (peace be upon him): “Actions are only by intentions.” [Sahîh al-Bukhârî and Sahîh Muslim]
However, the extra amount will be considered as voluntary charity that is eligible for Allah's blessings.
Source: Islam Today