Am I obligated to repay my brother the amount I borrowed from him in Sudanese pounds equivalent to its previous value in Saudi riyals (8500 riyals), or equivalent to its current value in Sudanese pounds to avoid falling into usury, knowing that our agreement was to repay in Saudi riyals?

You are not permitted to agree with your brother to repay Saudi riyals instead of the Sudanese pounds he lent you. This is because it is a currency exchange (musarafah) that requires immediate possession of both currencies and falls under the prohibited category of "loan and sale" (salaf wa bay'a). This agreement is void, and you owe your brother the equivalent amount of the loan in Sudanese pounds.

However, when it is time to repay, it is permissible to agree to pay the value of the amount in another currency, provided there is immediate possession (musarafah) at the place of transaction.

A change in the value of the borrowed currency is not taken into consideration as long as the currency is still in use and the change is minor. The principle for repaying debts is to return the like amount, not the value.