Is it permissible to sell a debt of 600 dinars for 400 dinars in cash, knowing that the debt is the remaining price of a car sale, and that the brother is the one who will buy the debt?

Selling a debt to someone other than the debtor is a matter of scholarly disagreement. Those who permit it stipulate that the debtor must acknowledge the debt, be solvent or have evidence against him, and that the exchange must take place in the same session if the item is not typically sold on credit. The scenario mentioned in the question is impermissible due to the inequality between the two عوضين (exchanged items/considerations), and it falls under the usury of the pre-Islamic era (riba al-Jahiliyyah), as the brother lent 400 for 600.