I heard a fatwa that said that zakat must be paid on the salary an employee takes every month, after he has paid for all his basic and important needs. Then he should pay 2.5% of whatever is left. How correct is this fatwa?

Our opinion is that zakat on salaries is the same as zakat on cash in general. It is due when a person`s wealth has reached the minimum legal amount (nisab) and a full Islamic lunar year (hawl) has passed, whether it reaches the nisab by itself or reaches it in combination with his other wealth. The obligatory amount to be paid is one-quarter of a tenth, as it is said in Arabic, or 2.5%. Paying zakat on monthly salaries in the way mentioned in the question is not the opinion of AMJA, even if some modern scholars have said this. In any case, whoever does this would have taken the safer way and we ask Allah to accept our deeds and his.