Is it permissible to sell mobile phones to customers by installment, knowing that I first buy them from the store for cash then sell them to them by installment?

It is not permissible for a partner or an agent to buy from himself unless he has the permission of his partner or principal; this is because he is accused of favoring himself. This is the view of the majority of jurists. Similarly, it is not permissible for a partner to exploit the premises of his partners and sell therein for himself without their permission, nor for an employee to work during his official hours for anyone other than his employer. The permission of the partners is required for selling to oneself, and for doing so inside the premises, with the partners' goods, and during working hours. If the partners give permission, and they are adult and of sound mind, then there is no harm. You may buy the device from the store for yourself, pay its price, and then sell it. However, if one of the partners is not of sound mind, then it is not permissible to act even if he gives permission, because his permission is not considered valid.