What is the ruling on fictitiously buying a car from an Islamic bank with the aim of obtaining money for marriage, then transferring its ownership to a brother who pays its price and thereby covers the marriage expenses with that money?

If the intended transaction is for you to buy your brother's car in installments through the bank, then sell it back to your brother for the cash price the bank paid him, while you pay the installments to the bank, then this transaction is not permissible. This is because it falls under the category of 'inah (buyback) transaction, where the bank is merely a facilitator, and the commodity returns to its original seller. This resembles an attempt to circumvent something disliked.