Are the profits of Islamic banks permissible or forbidden, and is it better to avoid them? And is it usury for the bank to purchase a commodity and then sell it in installments for more than its original price?

The profits of Islamic banks are permissible if they result from investing funds in legitimate ventures. Purchasing goods in installments through an Islamic bank is permissible if the bank owns the commodity, guarantees it, and then sells it to the customer at a known price, without stipulating a penalty for late payment. There is no harm in the bank selling the commodity at a higher price for installment payment, because time has a share in the price. However, if the bank pays the price of the commodity on behalf of the buyer and collects it from him with interest, then this is a forbidden usurious loan. Allah Almighty says: (Those who consume interest will not stand [on the Day of Resurrection] except as one stands who is being beaten by Satan into insanity. That is because they say, "Trade is [just] like interest." But Allah has permitted trade and has forbidden interest. So whoever has received an admonition from his Lord and desists may have what is past, and his affair rests with Allah. But whoever returns [to dealing in interest] – those are the companions of the Fire; they will abide eternally therein.)