What is the ruling on zakat for a monthly salary from which nothing remains until the end of the year, for a residential apartment, and for the wife's gold (mahr/dowry)?

There is no zakat on salary until it reaches the nisab, which is equivalent to the value of 85 grams of gold. It is a condition that a full year (hawl) passes on this money from the day it reaches the nisab, whether by itself or combined with other money owned by its proprietor.