Is a merchant's purchase of a phone from the very same buyer to whom he sold it on installment, before the installments are complete, considered the forbidden Bay' al-'Inah? And what is the ruling on taking the device to sell it as a commission, or considering it a down payment for a new device on installment?

There is no objection to purchasing a mobile phone from the customer after its value has changed and decreased, if it was previously sold to him in installments. This is an exception to the prohibited bay' al-'inah (buy-back sale), provided that the decrease in price is due to a change in the item's condition, and not for the purpose of accelerating the payment. It is also permissible for you to appoint someone to sell the mobile phone on behalf of the customer for a known and specified commission at the time of the agency contract. As for selling the old device to pay for a new device in installments, the ruling is as stated in the first point.