Must I pay the amount that satisfies my brother after 15 years have passed since the loan I did not repay, or should I give 5000 dinars in charity, especially since he refuses the original amount claiming a change in purchasing power?

The procrastination of a wealthy and capable person in repaying a debt is clear injustice. The creditor has the right to bring the matter before a judge to compel the debtor to pay. It is not permissible for the lender to demand more than what he gave, and the borrower must repay the equivalent of what he borrowed, not its value. If the debtor repays his debt and adds to it out of kindness, without any prior condition, then that is a commendable act. If the creditor refuses to accept the amount, the debtor's responsibility is discharged by enabling the creditor to receive it, and it is preferable to have witnesses to this.