Question: I give zakat on Shawwal 3 every year. Is there anything wrong with giving it in Ramadan this year before the passage of a complete lunar year? Is the amount given counted as optional charity if it is given before the due date?
It is permissible to give zakat ahead of time, and it is not counted as optional charity. If one remains below nisab when due date comes, only then will it be counted as optional charity.
When Shawwal 3 comes, you must calculate your zakatable assets again. If the amount you gave early is less than the amount you must give, then you must pay the balance. If it is more than the amount you must give, there is no harm. It is permissible, too, to give zakat for a few number of years in advance. (Tahtawi)
If one calculates ones zakatable assets wrongly and gives two gold coins instead of one gold coin and if one, after giving it to a poor person, calculates again later and sees that one must give one gold coin as zakat, then one may count the additional amount in next years zakat. (Nimat-i Islam)
Question: I must give three gold coins as zakat. Can I give five gold coins and count the additional amount in next years zakat?
Yes, you can.