What is incumbent upon one who has paid zakat on his wealth for five years [and later found out he was not obligated to]? And is it permissible to pay zakat by slaughtering a sheep?

Delaying the distribution of Zakat after a full year has passed is not permissible and necessitates repentance and prompt payment to its rightful recipients, based on the Almighty's saying: "And give Zakat," and the command is for immediate action according to the majority of scholars. As for paying Zakat in advance before a full year has passed, the majority of scholars permit it, and it is permissible to pay it in advance for two years only, and not for more than that unless there is an urgent need. Distributing Zakat by slaughtering a ram or similar is not permissible, because Zakat must be given from the same type of wealth on which Zakat is due, and monetary value is not sufficient according to the majority of scholars.