All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.

It is the duty of the person whose wealth is liable for Zakah (Obligatory charity) to ask the scholars about the exact amount of Zakah he has to give since Allah the Almighty says {What means}: " And before thee also the apostles We sent were but men, to whom We granted inspiration: if ye realise this not, ask of those who possess the Message." {An-Nahl, 43}.

Therefore, if he had paid more than the required amount of Zakah, then the excess amount is considered a voluntary charity for which he will be rewarded since Almighty Allah says {What means}: " And if any one obeyeth his own impulse to good,- be sure that God is He Who recogniseth and knoweth." {Al-Baqarah, 158}. Moreover, the giver has no right to ask the recipient of Zakah to give back the excess amount unless the former had told the latter, upon handing him the money, that it is Zakah, because the Zakah has entered into the possession of the latter. Imam An-Nawawi (May Allah have mercy on him) stated: "Sadaqa could be obligatory (Zakah) or voluntary (Charity), so if it isn`t obligatory then it is voluntary. For example, if someone gave Zakah of his wealth that was in another country, but found out that it was gone, then the amount that he had given is considered voluntary charity." {Al-Majmou` Shareh Al-Mohathab, V.6:183}. 

In addition, it isn`t permissible to consider that excess amount as  advanced Zakah for the next year, because the giver intended that amount as Zakah for last year and advancing Zakah for the next year requires a new intention since Zakah for each year is considered an independent act of worship. Thus, intention to give Zakah should be renewed each year, be it paid on due time or advanced.

Shafie jurists have stated that when a person gives the exact amount of Zakah due on a particular wealth of his, but realizes that he has given an excess amount (E.g. he found out that the wealth itself was gone), then it doesn`t avail him to intend that excess amount as Zakah for another wealth that he owns. Kindly check {Moghni Al-Mohtaj, V.2:130}.

In conclusion, the excess amount paid by mistake isn`t considered advanced Zakah for the next year, and the giver is allowed to ask the recipient to give it back if he had told him that the money he is giving him is Zakah. And Allah the Almighty knows best.