Is zakat obligatory on a deposit amount that reaches the nisab, from which installments for an apartment are paid, knowing that the profits are what pay the installments, not the principal amount, and that the deposit amount is less than the price of the apartment?

It is not permissible to deposit money in an interest-based bank except in cases of extreme necessity, and then only in a current account. Interest-based earnings must be disposed of by spending them on the needy or on public welfare. Zakat (definition) is obligatory on the saved amount if it reaches the nisab (definition) and a full year has passed over it. This is done by deducting debts from the existing money, then paying zakat on the remainder. Interest-based profits are not to be included in the money for which zakat is paid; rather, they must be disposed of.