An Agent Who Purchased a Product and Received a Gift: To Whom Does the Gift Belong?

You must split the value of the gift equally with your friend. According to halachah, when an agent receives a gift as a result of purchasing merchandise with the principal’s money, the agent and the principal divide the gift between them.
Sources and reasoning:

In the Gemara in Ketubot 98a, Rabbi Yehudah and Rabbi Yosei disagree regarding a case in which an agent purchased merchandise for his principal and the seller gave the agent additional merchandise without charging extra. To whom does this additional merchandise belong? Rabbi Yehudah holds that it all belongs to the agent, while Rabbi Yosei says that they divide it. The Gemara asks: another baraita states that Rabbi Yosei himself holds that everything belongs to the owner of the money. Rami bar Chama answers: there is no contradiction; one case concerns an item with a fixed price, and the other concerns an item without a fixed price. Rav Pappa likewise concludes: the halachah is that with an item that has a fixed price, they divide it; with an item that does not have a fixed price, everything belongs to the owner of the money.

It is clear that when a seller adds merchandise for the buyer without charging extra, if the merchandise has no fixed price—that is, it does not have a set market price—the addition belongs to the principal. However, if the merchandise has a fixed price—that is, it has a set market price—the agent and the principal divide the value of the addition. This is likewise the ruling of the Shulchan Aruch, Choshen Mishpat 183:6.

The early authorities disagree regarding the reason why the agent and principal divide merchandise that has a fixed price. According to Rashi and the responsa attributed to the Ramban, the reason is that there is doubt as to whom the seller intended to give the additional item as a gift: the agent or the principal. Therefore, they divide it. However, according to the Rif and the Rosh, they divide it because both are partners in bringing about the gift: the principal, because the merchandise was purchased with his money, and the agent, because he personally went and bought the merchandise in the store. The Rif writes: “Why does the agent divide it with the homeowner? Since the agent received this benefit through the homeowner, he divides it with him.”

In the case before us, whether according to the reasoning of Rashi and the Ramban or according to that of the Rif and the Rosh, the agent and principal must divide the value of the gift, since the intention of the store giving the gift is unknown—whether it intended to give the gift to the owner of the money or to the agent.

See Chashukei Chemed on tractate Ketubot 98a, where he was asked a similar question and wrote that the agent and principal must divide the value of the gift. This is also stated in Shamru Mishpat, vol. 2, siman 31.