Is selling goods through the aforementioned tender system—where an initial offer is presented to the customer based on the product's price and estimated delivery time before actually possessing the goods—considered selling what one does not own, and thus forbidden in Islam? And what is the Sharia-compliant alternative to it?

Supply contracts and tenders are not exempt from this ruling. Based on the decision of the International Islamic Fiqh Academy, if the subject of a supply contract is a commodity that requires manufacturing, it is an Istisna' (manufacturing contract) and its rulings apply. However, if it does not require manufacturing and is described in terms of liability, then the supplier is obligated to deliver it at the agreed time. Here, there are two methods: Either the importer pays the full price in advance at the time of the contract, in which case the contract is a Salam (forward sale) and is permissible under its conditions. Or, if the price is not paid in full, it is not permissible because it becomes a binding promise resembling the contract itself, thereby constituting a sale of debt for debt (bay' al-kali' bi-l-kali'), unless the promise is non-binding on either or both parties, in which case it is permissible provided that the sale is finalized with a new contract.

Since the customer's payment is made upon receiving the devices, which were not in existence at the time of the contract, the sale is impermissible. The Sharia-compliant alternative is for the purchase contract between you and the customer to be non-binding, but rather merely a promise to purchase without obligation.

Dr. Abd al-Wahhab Abu Sulaiman is of the view that a supply contract is not a form of selling debt for debt, and that the contract remains permissible and revocable until the delivered item is handed over. The implication of this is that the payment of the price is deferred until the goods are delivered, unless the buyer voluntarily chooses to pay in advance. It is advisable that this be done through a non-binding promise if the price is only to be paid upon delivery.