Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
There is nothing wrong in paying the Zakah on a monthly basis if the payment is in advance. This is the view of the majority of scholars.
In his response to your question, Prof. Dr. Monzer Kahf, Professor of Islamic Finance and Economics at Qatar Faculty of Islamic Studies, states:
Yes, Zakah can be paid in advance during the year. You can estimate the amount that will be due on you at the end of the lunar year (354 days) beginning from the day you acquired nisab (approx. US $ 1000).
This does not usually coincide with Ramadan. You then pay during the year any amount you like to pay, in equal payments or not, and at regular intervals or not, provided that when you give any payment, you keep in mind the intention that it is for your Zakah. You should also keep in mind that you pay the balance that may be due on you at the end of the year.
Elaborating more on the permissibility of paying Zakah on a monthly basis, Dr. Muhammad Bakr Isma`il, the late Professor of Exegesis of the Qur’an and Qur’anic Sciences at Al-Azhar University, adds:
Every committed Muslim should pay the Zakah at its time. However, a Muslim is permitted to pay the Zakah on a monthly basis under the condition that the payment in this case should be in advance. It is reported that the Prophet (peace and blessings be upon him) accepted the Zakah of his uncle Al-`Abbas two years in advance because of Muslims’ need for it at that time.
I would like to refer to the fact that some Muslim scholars oppose the aforementioned view. However, the great majority of them say that there is nothing wrong in paying the Zakah on a monthly basis. This latter view of the majority is believed to be the most correct, as Zakah is a right of the poor and hastening in paying people their rights is a meritorious act.
Almighty Allah knows best.