What is the ruling on selling mobile phones by installment, with delivery of the sold item after full payment of the price, knowing that I buy the mobile phone after receiving all installments from the buyer by a period of six months, thus obtaining the price difference in addition to the pre-agreed upon profits?

It is not permissible to sell a phone before owning it, due to the Prophet's (peace be upon him) prohibition of a person selling what he does not own, as he said: "Do not sell what is not with you." The permissible scenario is to sell a phone with specified characteristics, with delivery at a known future date, provided the buyer pays the full price at the time of the contract. This is known as "Salam" (forward sale). If the customer does not agree to this, then it is not permissible to sell the phone except after purchasing and taking possession of it, and then selling it in installments.