How is Zakat calculated on a sum of money that gradually increases throughout the year from one's salary: Is it on the balance that has been held for a full year (hawl), or on the final balance? And is Zakat obligatory on apartments prepared for rent or sale, or those whose intention has not yet been determined?

What you have saved from your salary: you pay zakat on the principal amount that reaches the nisab (minimum threshold) when a hawl (full lunar year) passes over it. Then, you pay zakat on each installment you benefited from when a hawl passes over it. This is the view of the majority of scholars. The Hanafis, however, hold that you pay zakat on all the money in your possession when a hawl passes over the original principal. This is easier and more beneficial for the poor, so choose what is easier for you.

Regarding the three apartments: Zakat is obligatory only on the apartment designated for trade. You assess its value at the beginning of each lunar year and pay one-quarter of one-tenth (2.5%) of its value as zakat. As for the other two apartments, there is no zakat on them because there is no firm intention of trading with them.

Regarding the money designated for paying installments: If you have assets that can be used to offset the debt, you pay zakat on all the money in your possession, including the money set aside for paying installments.