Is zakat obligatory on an amount of 500 riyals that was borrowed in 1997 and repaid in 2002? And what is the amount of zakat due, knowing that the local currency is dinar and the debt occurred during the Hajj season?

There is no zakat on a debt owed to you by an insolvent person or a procrastinator until you receive it. Once you receive it, pay zakat on it for one year. However, if the debt is owed by a wealthy person from whom you can collect it whenever you wish, then zakat must be paid on it every year. If you have not paid zakat on it in previous years, then you must pay zakat for those years, because zakat does not lapse with the passage of time.