If a person has a monthly salary and he spends all of it and has nothing left, so that at the end of the month he has spent all his money, then he does not have to pay zakaah, because for zakaah to be obligatory, one full year must have passed (i.e., one full year from the date of his taking possession of the nisaab, the minimum amount of wealth on which zakaah is due). On this basis, you do not have to pay zakaah, unless you save some of your wealth and that amount reaches the nisaab (threshold or minimum amount), and one year has passed.
With regard to the person who told you that the zakaah on a salary is like the zakaah on agricultural crops and you do not have to wait until one full year has passed before paying it, what he said is not correct.
As most people work for a salary, we think that it is a good idea to describe how zakaah is to be paid on salaries.
Zakaah on the salary of an employee:
The employee with a salary will find himself in one of the following two situations:
1 – Either he spends all of it and does not save anything, in which case he is not obliged to pay zakaah, as in the case of the person who asked this question;
2 – Or he will save some of it, sometimes more and sometimes less. So how is zakaah to be reckoned in this case?
The answer is: if he insists on having all his rights and on not giving any charity to those who deserve it apart from what he is obliged to give, then he should make a schedule of his earnings and write down every amount and the date on which he took possession of it. Then he should pay zakaah for each amount separately when one year has passed from the date on which he took possession of it.
But if he wants an easier method, and wants to be more generous and give precedence to the poor and others who are entitled to zakaah over himself, then he can pay zakaah on all the money he possesses when one year has passed from the date when his wealth first reached the nisaab. This will bring a greater reward and raise him higher in status; it is easier for him and is more generous towards the poor and needy and others who are entitled to zakaah. Whatever extra amount he may pay will be regarded as a “down payment” on the zakaah for any wealth for which one year has not yet passed.
(From Fataawa al-Lajnah al-Daa’imah, 9/280).
For example: a man takes his salary for Muharram, and saves one thousand riyals of it. Then he does likewise in Safar and the remaining months. When Muharram comes in the following year, he looks at all that he has and pays zakaah based on that.
And Allaah knows best.