Reference: Sharh ‘Umdatil-Fiqh – Tape No.7, Question No.32
Question: Is it permissible for me to buy a car for my friend, which he wants; so I [intend to] buy it outright for cash, and then sell it to him on a payment plan with profit [for myself]?
Response: So long as you own the car outright, then it is permissible for you to sell it to him at an agreed price – either immediately, or at a later date; there is no harm in that. [You must] own the car outright, and [only then can you] sell it to him if you wish.