Is it permissible to count a sum of money that was given as charity to the poor as Zakat after it has been handed over?

Intention (niyyah) is a prerequisite for disbursing Zakat because it is an act of worship and an offering of devotion. The intention must accompany the act or precede it by a short period. Therefore, if someone gives a charity with the intention of it being a voluntary Sadaqah, and then wishes to count it as Zakat, it will not suffice, based on the Prophet's (peace be upon him) saying: "Indeed, actions are by intentions." Voluntary charity does not substitute for obligatory Zakat, and if one does not pay it with the intention of Zakat, the obligation does not drop.