The proof is twofold:

1) If Matanah Al Menas l'Hachzir is only a Kinyan li'Zman, then even after Rebbi Yehoshua (1st recipient) gave the Esrog to Rebbi Akiva (2nd recipient) with Matanah Al Menas l'Hachzir l'Raban Gamliel, Rebbi Yehoshua would not have to make a new Kinyan in order to regain the rights he originally received from Raban Gamliel. He never forfeited those rights; he simply gave them as a Kinyan li'Zman to Rebbi Akiva, and he received them back upon the conclusion of Rebbi Akiva's Mitzvah.

Since Rebbi Yehoshua is still considered to be the "borrower" after Rebbi Akiva's Mitzvah we should look at it as though he asked Rebbi Akiva to be his (Rebbi Yehoshua's) Shali'ach to return the Esrog to Raban Gamliel and we should say Shelucho Shel Adam k'Moso.

It must be that Matanah Al Menas l'Hachzir is a full-fledged Kinyan, and Rebbi Yehoshua never received back the Esrog after he gave it to Rebbi Akiva. Therefore, Rebbi Akiva cannot be considered a Shali'ach returning Rebbi Yehoshua's Esrog; he is returning his own Esrog.

2) The phraseology of Rav Avigdor Cohen (who maintains that Matanah Al Menas l'Hachzir is not a full Kinyan but a Kinyan li'Zman), as quoted by the Avnei Milu'im there, implies that the words "Al Menas l'Hachzir" are not emphasizing the positive ( return it), but the negative ( do not eat it or destroy it before it gets back to me). If the object is not destroyed before it reaches the hands of the giver, the condition has been fulfilled, no matter how it gets back to the hands of the giver. If so, Tosfos question clearly does not start. He must hold like those who argue and say that the main emphasis is on the act of returning the object. Since Rebbi Yehoshua did not do such an act of returning, Tosfos thought that he did not fulfill the condition.

Both proofs from Tosfos can perhaps be refuted, but they are strong evidence of Tosfos' opinion nonetheless.

M. Kornfeld