Alms Tax: Giving Zakat Before it Becomes Obligatory
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1983
If before zakat becomes obligatory for a person he gives something to the poor as zakat, he cannot count it as zakat. However, if afterwards when zakat becomes obligatory for him the thing that he gave to the poor has not perished and the poor person has remained poor, he can count the thing that he gave him as zakat.
-Ayatullah Sistani, Practical Laws of Islam, Alms Tax (Zakat)