Shekalim, Chapter 2, Mishnah 4. In the previous Mishnah we learned that if a person collected money for his half-shekel, according to Beis Shammai the surplus is chullin (ordinary, unconsecrated money), since he intended to consecrate only the half-shekel itself, and whatever accumulated beyond that amount, perutah by perutah, remains chullin. By contrast, one who collected perutos in order to buy a chatas (sin offering) that he is obligated to bring, the surplus goes to nedavah, and even Beis Shammai agree that olos are brought with it in the Beis HaMikdash. The question that arises is: what is the difference between the two cases?
Rabbi Shimon's question:
Our Mishnah opens with this very question. "Mah bein shekalim l'chatas?" - what is the difference between one who collects money for the half-shekel, where the surplus is chullin, and one who collects money to buy a chatas, where the extra money has the status of hekdesh and is used to purchase offerings brought as olos?
To this the Mishnah responds:
"Shekalim yesh lahem kitzvah" - the shekalim have a fixed, set amount: a half-shekel. By the very definition, a person knows in his own mind that anything beyond that amount is not included in it and remains chullin.
"V'chatas ein lahem kitzvah" - a chatas has no fixed amount. One can spend a large sum on the animal and one can spend a small sum, and it was within his ability to buy a more expensive animal. For this reason, the fact that he bought a cheaper animal does not mean that the money left over is chullin; that money had the potential to be part of the chatas, and therefore we are stringent with it and treat it as hekdesh, and it goes to nedavah, to purchase offerings brought as olos.
Rabbi Yehudah's objection:
Rabbi Yehudah responds to this: the shekalim too have no fixed amount, meaning that the half-shekel does not truly have a set measure. He brings proof for his words from history: "She'k'she'alu min ha'golah hayu shoklim darkonos, chazru lishkol sela'im, chazru lishkol tiv'in, u'vikshu lishkol dinar v'lo kiblu mehem" - these are the stages of how matters developed:
Darkonos: when Israel came up from Bavel to build the second Beis HaMikdash, they gave darkonos as their contribution toward the communal offerings - a gold coin worth four times the half-shekel - since they needed a great deal of money.
Sela'im: afterward, when they had more money available, they began to pay a sela - a full shekel, double the amount actually required.
Tiv'in: once the people multiplied and the funds multiplied, each individual no longer needed to pay so much, and they began to give a tiva as their half-shekel, which is exactly a half-shekel.
Dinar: they wanted to go down further, to a dinar - a smaller coin - and this they were not permitted to do, since it is less than the minimum amount the Torah set, a half-shekel, and at that amount they had to remain.
This is Rabbi Yehudah's claim: with the shekalim too the amount is more open ended, and therefore the same ruling as the chatas should apply to them as well - that the surplus should be hekdesh, since it was within his ability to give more.
Rabbi Shimon's answer:
To this Rabbi Shimon responds: "Af al pi chein, yad kulan shavah" - at every single stage, even when a larger amount was set because a great deal of money was needed, everyone gave the very same amount. It is not that one person gives this amount and his fellow gives a different amount; rather, at any given time there is a fixed amount for everyone. For this reason, anything beyond that amount is not hekdesh but chullin, as explained in the Mishnah.
Not so with the chatas: "Zeh meivi b'sela, v'zeh meivi bishtayim, v'zeh meivi b'shalosh" - at one and the same time different people bring different amounts; one person spends a sela on his animal, another two sela'im, and another three. Consequently, the money left over beyond what was actually spent on the animal could potentially have been used for the chatas, and therefore it has the status of hekdesh and is given to nedavah, to bring olos in the Beis HaMikdash.
In summary: our Mishnah discusses the reason for the difference between the surplus of the shekalim and the surplus of the chatas. According to Rabbi Shimon, the shekalim have a fixed amount and therefore the surplus is chullin, whereas the chatas has no fixed amount and therefore the surplus is hekdesh. Rabbi Yehudah responded that the shekalim too have no fixed amount, as is proven from the changes that were practiced from the time they came up from Bavel - darkonos, sela'im and tiv'in, and their request to weigh out a dinar, which was not accepted. Rabbi Shimon answered that even so, everyone's contribution was equal: in every period all gave a uniform amount, whereas with the chatas different people bring different amounts at one and the same time.
With this, the dispute between Rabbi Shimon and Rabbi Yehudah in our Mishnah is clarified, along with the underlying difference between a fixed amount and a sum that has no fixed amount.