What is the ruling on a bank selling a commodity by installments, a commodity it does not own until after a buyer requests it, whereupon the bank purchases it from the original seller and then sells it to the buyer? And does this fall under "the Messenger [PBUH] forbade two sales in one transaction"?

Murabaha to the one who promises to purchase (Murabaha lil Amiril Bishira) is permissible under certain conditions, among them that the promise to purchase is not binding. There is nothing in Sharia (definition) that prohibits this transaction, as it falls under the Almighty’s saying: (And Allah has permitted trade and forbidden interest). Nor does it prevent it, according to Sharia, that the bank did not own the commodity until after the request of the one promising to purchase. Imam Shafi’i mentioned that a man buying a commodity based on a request and a specified profit is permissible, and that the saying that selling before the seller takes possession or at risk is forbidden is an ijtihad (definition) (juristic reasoning) and not a Sharia text. The reference is the Book of Allah and the Sunnah (definition) of His Messenger, and there is nothing in them that prohibits this transaction.