In a Murabaha sale, does the client ask the bank to purchase for him the commodity he desires at a known cost and agreed-upon profit, or must the commodity be owned by the bank beforehand and then sold by the bank to the client in installments?

There is no difference in Murabaha sales whether the commodity is owned by the seller (the bank) or if the seller acquires it based on the customer's request. If the seller takes possession of the commodity and receives it, then it is permissible to sell it to the customer.