Does the free coupon the purchaser received when buying his friend's phone for an agreed-upon price belong to him or to his friend?

Since the brother who asked the question bought the phone for his friend, he is acting as his agent. The rights of the contract are tied to the principal, and ownership is transferred to him. He is responsible for the price, defects, and guaranteeing the item, and so on. Therefore, the offer that accompanies the purchase belongs to the principal, not the agent.