All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
To give Zakah in installments has two cases:
First: to give it in installments before it becomes due, i.e. to give it in advance before the end of the lunar year, like the one who gives a poor family 5000 monthly with the intention to make it Zakah, and at the end of the lunar year, he calculates the whole amount of Zakah due on him: if it is more than 60,000, he gives out the remaining sum; and if it is less than that, he considers the extra sum voluntary charity, which has a reward with Allaah. This kind of giving in installments is permissible.
Second: to give Zakah in installments after it becomes due; and it is impermissible unless there is a preponderant benefit or a legitimate excuse.
As for the Zakah due on you for the years that have passed, you should pay it immediately and without laxity, because it is a debt due on you to the poor, which you should give to them. To delay giving it is unlawful procrastination, about which the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said: “The procrastination of the wealthy (to pay the debt) makes lawful his punishment and (violable) his honor.” [An-Nasaa’i, Abu Daawood, Ibn Maajah and others]
Allaah Knows best.