it becomes due, provided he possesses the nisab. If he does not possess the nisab and says, “I will pay my zakat in advance, because wealth will come to me in the future,” his payment is not valid, because he has paid it before the cause of its obligation—namely, possessing the nisab.
This is based on a principle mentioned by Ibn Rajab, may Allah have mercy on him, in al-Qawa'id al-Fiqhiyyah: “Doing something before its cause is ineffective, whereas doing it before its condition is permissible.” (1)
For example, if a man possesses 190 dirhams and says, “I wish to pay zakat on 200,” this is not valid, because he has not completed the nisab; thus, the cause does not exist, and doing something before its cause is invalid.
However, if he possesses the nisab and pays it before the completion of the lunar year, it is permissible, because he has done so after the cause but before the condition; for the condition of zakat becoming obligatory is the completion of the lunar year.
A comparable case is that of someone who offers expiation for an oath he intends to take before actually taking the oath, then takes it and breaks it: the expiation is not valid, because it was offered before the cause. But if he takes the oath and offers expiation before breaking it, the expiation is valid, because he offered it after the cause but before the condition.
The evidence for the permissibility of paying zakat in advance is both textual and rational. As for the textual evidence: Abu 'Ubayd narrated in al-Amwal, with his chain of transmission, from Ali that the Prophet, peace and blessings be upon him, collected from al-Abbas the zakat of two years in advance. (2)