Is taking the agreed-upon interest considered a sin, and must it be returned to the partner after having previously relinquished it for the purpose of helping him?

It appears that the questioner's statement that he did not want profit from selling his share was merely a promise to encourage the purchase. Therefore, he is not obligated to fulfill it if it becomes apparent to him that it is not in his interest, whether he intended to leave his profit for his partner or give it to his uncle. It is, however, recommended for one who makes a promise to fulfill it.