1) The Gemara states that according to Rav Huna brei d'Rav Yehoshua, the reason why the prepaid taxes cannot be recovered is "Hini'ach Ma'osav Al Keren ha'Tzvi" -- "he has placed his money on the horns of a deer." In other words, he has thrown his money away and will not get it back.

Rav Huna is referring to the Mishnah in Kesuvos (107b) where Chanan says that if someone paid the food bill of a woman whose husband went overseas, he has lost his money since he did not make any agreement in advance that the husband would pay him back. Rashi (Kesuvos 108a, DH Chanan) writes that the same applies for any debt that was paid up without the agreement of the debtor. This Halachah is recorded in the Shulchan Aruch, Choshen Mishpat 128:1.

2) Therefore, when Gidal paid three years in advance even though he had no need to do so, this was like placing the money on the deer's horns. This is stated by the Ritva here: "k'Pore'a Chovaso Shel Chavero she'Lo mi'Da'ato v'Hini'ach Ma'osav Al Keren ha'Tzvi." Gidal had paid up another person's debt and forfeited his money.

3) According to this, what did Rav Papa think before Rav Huna's challenge? Why did he not agree with this explicit Halachah that Gidal had lost his money? Rav Yosef Shalom Elyashiv zt'l, in his He'oros on Maseches Gitin, cites a Teshuvah of the Maharit (1:124) who quotes the Talmud Yerushalmi (Kesuvos 13:2) that the reason why one who pays up his friend's debt cannot recover his money is that the friend can argue that he would have persuaded the creditor to forgo the debt. The Maharit notes that this is a rather far-fetched argument. However, he explains that the idea behind this Halachah is that since the debt was paid up without the knowlege of the debtor, this means that the payer was not acting according to Halachic norms and seemed simply to be doing a favor for his friend. If so, the debtor can argue, even with quite a weak argument, that he is not obligated to refund the money.

4) Rav Elyashiv said that according to this, we now can understand what Rav Papa thought originally. In this case, Gidal thought that the fields were going to be in his possession for three years, because the owners had run away and there was no sign of them coming back in the near future. Therefore, Gidal argued that he was paying the tax, not as a present to the original owners but as his legal duties to the king, and consequently the Din that we cited above (1) would not apply here.

5) Rav Huna disagreed with this because, after all, it was possible that the owners might return any day and take their field back, so Rav Papa had no need to pay up the entire three years' sum, and therefore he was considered as though he placed his money on the deer's horn.

(I have deliberately not gone into the issue of "Sikrikon," which is, of course, the reason why the Gemara mentioned this story here in the first place, because I do not want to complicate matters.)

Kol Tuv,

Dovid Bloom