(Mishnah): If a person was riding on an animal and he saw an (ownerless) item and told his friend to go and take it for him and the friend took it and said that he had acquired it for himself, the friend acquires it.

If after the friend gave it to him he claimed that he had acquired it originally for himself, he is not believed.

(Gemara - R. Yitzchak): Neither the beginning nor the end of the following case teaches us anything (about the question of whether a person is able to lift up an ownerless item in order to acquire it for someone else) - The first law of the Mishnah says, "If a person was riding on an animal and he saw an (ownerless) lost article and told his friend to go and take it for him and the friend took it and said that he had acquired it for himself, the friend acquires it.'' - if he would have said 'acquire it for me' rather than 'take it for me', it might have worked. The second law of the Mishnah says that after he gave it, he cannot claim that he took it for himself, but nevertheless, even if he had not yet given it, perhaps he could not have actually acquired it for the sender. Therefore this Mishnah cannot prove anything about the question of acquiring for someone else.