Is zakat obligatory on a sum of money on which zakat was paid last year before the end of its hawl (zakat year) on a portion of it, then this year, two days before the zakat due date, the sum was used to purchase an apartment which resulted in a debt with monthly installments? And is it permissible to give zakat to a brother who is studying at university and has no source of income?

Zakat (definition) is due on the remaining amount if it reaches the nisab (definition) (minimum threshold) and a full hawl (lunar year) passes over it before the apartment is purchased; otherwise, it is not due. If the apartment is for residence, there is no zakat on it. However, if it is for sale, zakat is due on its value after a full hawl passes over it. If it is for rent, zakat is due on its rental income after a full hawl passes and it reaches the nisab. Installments due are deducted from the zakatable wealth if zakat becomes obligatory. If what remains reaches the nisab, zakat is obligatory; otherwise, it is not, unless there is other wealth exceeding basic needs.