What is the ruling on selling the free vitamin bottle that the questioner received as part of a "buy one, get one free" offer?

Since the friend was appointed as an agent for the purchase, the agent is entrusted and must inform the principal of the price at which she bought the item. Any gift received from the seller belongs to the principal, and the agent does not have the right to sell it. This is based on Al-Bahuti's statement in Kashshaf al-Qina: "If the agent buys a sheep worth a dinar for less than that, the purchase is valid. And the surplus belongs to the principal." He substantiates this with the Hadith (definition) of Urwah ibn al-Jad, who narrated that the Prophet (peace and blessings be upon him) sent him with a dinar to buy a sacrificial animal for him. So, he bought two, sold one for a dinar, and brought the other to the Prophet (peace and blessings be upon him).