OPINIONS: The Gemara explains that in the times of Moshe Rabeinu, the Sela was worth 20 Ma'in. Later, the Chachamim decreed that the Sela is to be defined as 24 Ma'in, thereby making the Sela more valuable.
Why may the Chachamim tamper with the Torah's Shi'urim? If the Sela of the Torah is 20 Ma'in, then what right do the Chachamim have to alter that Shi'ur? We understand that the Chachamim can enact a decree requiring us to give more money to the Kohen for Pidyon ha'Ben, but how can they change the Shi'ur of the Sela, such that one does not fulfill the Mitzvah even mid'Oraisa if he gives five Sela'im according to the original Shi'ur?
(a) RASHI (DH Basar) implies that the Torah itself gives the Chachamim of each generation the right to define what is considered a "Sela." When the Chachamim re-define the Sela, all of the Torah's monetary Shi'urim change with that modification. This seems to be the approach of the other Rishonim as well (see TOSFOS to Kesuvos 10a, DH Nosen).
(b) However, TOSFOS in Kesuvos (ibid.), citing the Yerushalmi, suggests that the Gemara here may mean something else entirely. Perhaps the Sela of the Torah is defined by the coin with the largest base denomination (that is, the largest single-unit coin) of the country (for example, the dollar in the United States, the pound in the United Kingdom, the peso in Mexico, the yen in Japan, etc.). If the coin is large (i.e. more valuable), then the amount required for Pidyon ha'Ben in that country is larger than in other countries. If the coin is smaller than a Sela of the Torah, then Pidyon still requires a Sela (since, as the Gemara says, one may add to the value of the Shekel but not subtract from it).
The RAMBAM (Hilchos Shekalim 1:5-6) rules as Tosfos suggests with regard to the Machatzis ha'Shekel that is collected each year on Rosh Chodesh Adar for the public Korbanos of the coming year. In a time when the coin commonly used is larger than a Shekel, one must give more than a half-Shekel. (With regard to Pidyon ha'Ben, however, no mention of a variable fee is made by the Rambam or by the Shulchan Aruch.)
QUESTION: The Gemara relates that Rav Ashi sent seventeen Zuz (Dinar) to Rav Acha brei d'Rava in order to redeem his son. Rav Ashi sent a message to Rav Acha asking him to return to him change of a third of a Dinar. RASHI explains that since Rava said that a Sela of the Torah is worth 3 1/3 Dinarim, the five Sela'im that must be given for Pidyon ha'Ben are 16 2/3 Dinarim. Therefore, Rav Ashi asked for one third of a Dinar in return as change.
Rav Acha replied that, on the contrary, Rav Ashi should send him an additional three Dinarim, because the Chachamim added to the Sela, making it worth four Dinarim (instead of 3 1/3). Therefore, Rav Ashi needs to send him an additional three Dinarim.
It is evident from this incident that it is possible to perform Pidyon ha'Ben through a Shali'ach, an agent. Accordingly, this Gemara contradicts the ruling of the REMA (YD 305:10) in the name of the RIVASH, who writes that the father cannot use an agent to redeem his son. How does the Rema understand this Gemara? (See MAHARIT ALGAZI #79.)
ANSWER: The CHASAM SOFER (Teshuvos, YD #297) explains that there are different levels of agents. The type of Shali'ach that one appoints in order to marry or divorce a woman on his behalf is very different from the type of Shali'ach that one appoints to redeem his son. When a man is Mekadesh a women through a Shali'ach with Kesef (money), in addition to the requirement that the money or object of value must reach the woman through the Shali'ach, the Shali'ach is required to proclaim, "With this money (or ring) you are hereby married to the one who sent me." When the Shali'ach makes this statement, it is considered as though the husband himself said it. For marriage and divorce, the hand of the Shali'ach is considered equivalent to the hand of the husband.
Such a degree of Shelichus is not necessary for Pidyon ha'Ben. In the case of the Gemara here, it would have sufficed to place the five Sela'im (together with a note informing the Kohen that it is being sent for Pidyon ha'Ben) on a monkey trained to bring it to Rav Acha. There is no requirement to make any declaration upon giving the money to the Kohen at the time of the Pidyon, as long as the Kohen knows that he is receiving the money for Pidyon ha'Ben.
The Rema, in contrast, is discussing a case in which a man asked a Shali'ach to redeem his firstborn son with the Shali'ach's own money. The Shali'ach did not transfer (Mezakeh) his money to the ownership of the father, but instead he gave his money to the Kohen with intention to redeem his friend's son. (This is similar to one who separates a portion of his own produce as Terumah on behalf of someone else's produce.) In such a case, where the Shali'ach wants to perform the actual Pidyon, it is clear to the Rivash and Rema that such a Shelichus is not valid for Pidyon ha'Ben. However, when the father sends his own money with the Shali'ach, it is not considered as though the Pidyon ha'Ben was performed by a Shali'ach, but rather merely the transporting of the money was done by a Shali'ach. The Pidyon itself was effected by the father through sending his own money to the Kohen. (D. BLOOM)