Does mere doubt about an institution's involvement in usury prevent opening an account with it, and is it permissible to rely on the seller's announcement that the price of the commodity does not increase with installment payments, even if there is an intermediary who charges fees?

Installment sales are permissible even if the installment price is greater than the spot price, provided that both parties agree upon a specified and known price before the contract. It is not permissible to increase the price after the contract due to an extension of the payment period. The presence of an intermediary who facilitates the sale for a known fee is permissible. However, the prohibited aspect is for the intermediary to pay the price to the seller and then recover more than what he paid from the buyer, as this is considered usury (riba) and is forbidden.