Shekalim, Chapter One, Mishnah Four
In this mishnah we see two early tannaim arguing over whether or not a priest donates the half-shekel.
1) Rabbi Judah said: Ben Bukri testified at Yavneh that a priest who paid the shekel is not a sinner.
2) But Rabban Yohanan ben Zakkai said to him: not so, but rather a priest who did not pay the shekel was guilty of a sin, only the priests expounded this verse for their own benefit: And every meal-offering of the priest shall be wholly burnt, it shall not be eaten (Leviticus 6:16), since the omer and the two loaves and the showbread are [brought] from our [contributions], how can they be eaten?
Section one: Ben Bukri assumes that a priest is exempt from the shekel offering. The midrash which explains this appears below, in section two. His testimony is that despite the fact that the priest need not donate the half-shekel, if he does donate it he has not transgressed. We might have thought that person who is not liable to pay the half-shekel may not donate it. The problem with a voluntary donation of the half-shekel is that public sacrifices must come from the entire public, meaning from the half-shekel. A voluntary donation may be seen as an individual paying for a public sacrifice. Ben Bukri testifies that we dont perceive of the priests half-shekel in that way. Rather it is a gift to the community, which belongs to the community as a whole. As such it may be used to purchase public sacrifices.
Section two: Rabban Yohanan ben Zakkai reasons the exact opposite from Ben Bukri. He holds that the priest is liable to donate the half-shekel and if he doesnt do so, he transgresses in much the same way that any person who doesnt give the half-shekel transgresses. The priests tried to use some midrashic reasoning to get out of giving the half-shekel. The Torah states that any minhah, a meal-offering, given by a priest, must be wholly burnt. The half-shekel is used to purchase certain meal offerings, namely the omer (the barley offering brought between Pesah and Shavuot), the two loaves brought on Shavuot, and the weekly showbread. All of these are eaten by priests and not burnt. The priests claim that the fact that these are eaten proves that the priests did not pay for any of them, for had they paid for them they would have had to have been wholly burnt. Rabban Yohanan ben Zakkai says that the priests midrash is mistaken. Only individual minhah offerings of the priest are wholly burnt. Public minhah offerings paid for partly by priests may be eaten. Hence the priests are liable to pay the half-shekel.
We should note that aside from the technical aspects of this debate, there may be an underlying social/religious issue. The question is, are the priests a part of the people or are they a separate class, with their own unique relationship to God? This might be an interesting way of examining Jewish religious leadership in generalare leaders a part of the Jewish people, or are they a class on their own. Rabban Yohanan ben Zakkais answer would seem to be clearthe priests must give their half-shekel, they are part of the Jewish people and not above, or even truly separate from the rest.