Is zakat obligatory on an amount of money that I lent to my brother and he has not repaid it to me yet, knowing that he does not deny the debt, but the procrastination of merchants prevents him from paying it back?

If the money paid was a loan, then the stipulated increase is not permissible, because it is usury (riba). Only the principal amount should be returned to you. Its zakat becomes obligatory upon its receipt for one year, or for all past years. However, if the money was a partnership, then stipulating a guarantee of the principal is not valid, and its zakat is also obligatory. But if the debtor is in straitened circumstances or procrastinating, then its zakat is paid upon its receipt for one year.