Answer
(Fatwa: 826/522/L=1432)
If a person who is eligible to receive zakah was given some money without an intention of zakah, you can make intention of zakah until the amount is in his hand. But, if he spent the amount, now making intention of zakah will not be counted:
وشرط صحۃ ادائھا نیۃ مقارنۃ لہ ای للأداء ولو کانت المقارنۃ حکما کما لو دفع بلا نیۃ ثم نوی والمال قائم فی ید الفقیر (الدر المختار مع الشامی 3:187)
The zakah amount can be paid throughout the year. If you pay monthly and calculate at last, it is alright:
ویجوز تعجیل الزکاۃ ولا یجوز قبلہ کذا فی الخلاصۃ (عالمگیری 1:194)
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband