How is the zakat of a monthly salary paid if it reaches the nisab every two consecutive months and a hawl (lunar year) has passed over it, and is zakat obligatory on the amount for which zakat was paid last year and over which a hawl has passed again?

Salary is not subject to zakat unless a hawl (full lunar year) has passed on it, and it reaches the nisab (minimum threshold) either by itself or by being combined with other money or trade goods. The beginning of the hawl can be determined by two methods: The first is to make a separate hawl for each amount, but this is difficult. The second is to pay zakat on all savings when the hawl of the first nisab you owned is due, and this has greater reward. The hawl is calculated by the lunar year, not the solar year. If a hawl passes on zakat-eligible wealth and it has reached the nisab, then zakat becomes obligatory on it, except for produce from the land, which is subject to zakat only upon harvest.