Is the amount paid to the seller, which the seller stipulated would not be refunded to complete the sale with the bank, considered usury or a form of deception, especially since it was not part of the agreed-upon price?

This transaction, which is Murabaha for the one who commands the purchase, is permissible in general, with legal conditions. Among these conditions is that there should be no prior contractual relationship between the client who commands the purchase and the original seller of the commodity. The cancellation of any prior contractual relationship between them is a condition for the valid execution of the Murabaha process for the institution, so that the transaction does not devolve into an usurious loan. Accordingly, you should not have given any money to the original seller of the house, nor should you have accepted the completion of the sale between the seller and the bank on the condition of recovering the amount paid. However, since the transaction has already taken place and the bank was unaware of the agreement, the bank's purchase of the house and its sale to you are valid.