What is the ruling on exchanging a new phone battery for an old one after reaching an agreement with the phone's owner?

It appears that the questioner was an agent for his friend in purchasing the phone. An agent must act in the best interest of his principal and is not permitted to take anything for himself except with the principal's knowledge and permission. If he informs his friend of the true situation—that he will take the new battery for himself and give him an old one in return—then there is no harm in it if his friend agrees. Otherwise, he is not permitted to do so.