Is the refund received for a bedroom that was not delivered considered usury, given that its price was paid to the bank through Murabaha? And what should be done with this money if it is usury?

It is permissible for the buyer of a commodity on a Murabaha (cost-plus profit) basis to sell it to someone other than its original seller at any price after taking possession of it. However, it is not permissible to sell it back to the original seller at a lower price because that constitutes prohibited 'Aynah (a type of re-sale transaction to circumvent interest).

Based on this, your brother may sell the bedroom set to a third party at the price the bank purchased it for, provided he has taken possession of it. If he has not taken possession of it, the sale is void and must be annulled, based on the Hadith of Zayd ibn Thabit, who forbade selling goods before acquiring them. Malik limited this prohibition to food items if they were part of a sale.

Furthermore, the initial transaction might become void if the bank did not take possession of the furniture in a manner considered valid by Islamic law. If the transaction is void, it must be annulled by taking back the commodity and refunding the price. However, if this is not possible, it is permissible to follow the opinion of those who permit selling before possession for items other than food.