Is it permissible or impermissible to withdraw money in Dirhams from a brother's Visa card and send it to him in US Dollars to benefit from the exchange rate difference?

If your brother has an account in an Islamic bank branch, from which he withdraws money that is not a forbidden loan, then there is no harm in him withdrawing the money nor in you cooperating with him in that, and it is permissible for you to benefit from what he gives you. However, if his account is in an interest-based (usurious) branch, then it is not permissible to deposit money in it or deal with it except out of dire necessity. Your brother's purpose for this transaction is not a necessity that permits it, so do not cooperate with him and advise him to withdraw his money and deal with Islamic banks.