Is zakat obligatory on the loan amount repaid to a brother in installments, and how is it calculated?

You both must repent to Allah if the bank from which the loan was taken was usurious. The money became your possession as soon as you received it from your brother. You are required to add this money to what you own in cash and trade goods and pay Zakat (definition) on it if it reaches the nisab (definition). It is preferable to pay Zakat on all money without deducting debt, which is the view of Shafi'i and the choice of Ibn Baz and Ibn Uthaymeen. The majority view is to deduct the debt first and then pay Zakat on the remainder. Malik's madhhab (definition) is that if someone has a possession for personal use that they can dispense with, they should use it to offset the debt and not deduct it; otherwise, they should deduct the debt and pay Zakat on the remainder. If Zakat became obligatory upon you due to owning the nisab free from debt, then you must pay Zakat for the years for which it has not been paid. The amount of Zakat is a quarter of a tenth (2.5%), calculated by dividing the money by forty.