Is it permissible for me to take the price difference as a commission for buying a phone on behalf of another person, with the intention of doing so at the time of purchase, knowing that I will donate the amount to charity, or must I return it to its owner?

You are not allowed to take the excess amount resulting from purchasing the phone at a lower price than agreed upon, even if you intend to donate it or if you were keen on buying at the best prices. Rather, you must return the price difference to your friend, because an agent must strive for the best interest of his principal, just as Urwah ibn Al-Ja'd Al-Bariqi did with the Prophet, peace and blessings be upon him.