What happens if someone sets aside more money for his half-shekel than is required?
According to the Mishna on our daf, Bet Hillel rules that the money is not holy, while according to Bet Shammai it will be used for voluntary sacrifices, since money set aside for holy purposes cannot be returned. Bet Hillel agrees that if more money is set aside for a sin-offering than is necessary then the extra money will be used for voluntary sacrifices.
Rabbi Shimon explains that Bet Hillel distinguishes between shekalim and the korban hatat (sin offering) because of the passage describing the shekalim (Shemot 30:15) which teaches that a rich person cannot give more, nor a poor person less. Therefore, the shekel is a fixed amount and the individual who sets aside money for his shekel does not mean to give more than is necessary. The sin-offering, on the other hand, can cost any amount of money. Rabbi Yehuda objects to this, arguing that there is no fixed amount for shekalim, either. He points to different periods in history during which time different amounts were given as mahatzit ha-shekel (half-shekel). Rabbi Shimon’s response is that even during those periods there was an agreed upon, set amount that everyone had to donate.
In his argument, Rabbi Yehuda describes the various periods during second Temple times, when the Jews returning from exile first brought darkonot, then sela’im, then teva’im and finally, dinarim, which were rejected because their value was too small.
The Ra’avad explains the story as follows. When the Jews first returned to Israel from the Diaspora there were few people and the needs of the Temple were great, so the people brought large, more valuable coins as mahatzit ha-shekel. As time went on the donations were made smaller, until they reached teva’im, which were equal in value to the required half-shekel. When people wanted to bring an even smaller coin it was rejected, since the minimum amount that could be brought was the value of a half-shekel.
The Rambam interprets this story differently, due to a different understanding of the mitzva of mahatzit ha-shekel. According to him, the requirement is to bring one-half of the common currency of the time. Rabbi Shimon in the Mishna is describing that the currency changed over time and that the amount of money changed together with the coin that was in general use. When the common currency became teva’im the people had to give a whole coin, since half of that coin would have been less than the Biblical half-shekel, which is the least amount that can be given.
(See Rambam Hilkhot Shekalim 1:5-6 and the Ra’avad there.)