How is the zakat paid for an apartment sold in installments over five years, knowing that the buyer is able to pay the installments on time but does not possess the full amount if demanded immediately?

The most preponderant opinion is that zakat on debt is obligatory upon the creditor in all cases. Thus, the installments owed by the apartment buyer are obligatory upon you. You can delay paying their zakat until you receive them, then pay zakat for the past years. Each installment you receive, you pay zakat on it for the period it was owed by the buyer. This is if the apartment was not for trade; otherwise, the hawl (one lunar year) for the installments is the hawl of the principal sum. The person mentioned is not considered indigent, as an indigent person is one who is unable to repay their debt.