Shekalim, chapter 1, mishnah 6. This mishnah introduces us to the kalbon - a small additional sum of money that was sometimes added to the half-shekel. Our mishnah presents a dispute between the first Tanna and Rabbi Meir regarding the reason for the kalbon and its purpose, and from this dispute there emerges a practical difference in halachah: when the additional sum was added and when it was not.
The two views on the reason for the kalbon:
The view of Rabbi Meir (cited at the end of the mishnah) - every time a person gives a half-shekel he is obligated to add a kalbon. The reason: to make certain that he has in fact given a full half-shekel, since perhaps the coin is not worth the true shekel, and therefore a little extra is added. It follows that with every giving of a half-shekel, in every case, a kalbon must be added.
The view of the first Tanna - the only purpose for which a kalbon is added is as a substitute for the money-changer's fee, that small commission a person pays a money-changer when he breaks a coin and receives change. Therefore the kalbon is relevant only when two people together give a whole coin - a sela - instead of each of them bringing his own half-shekel. Since their true obligation is to give a half-shekel, and had they broken the sela into half-shekels they would have had to pay a commission for the exchange, that same commission must be added when a person gives on behalf of himself and on behalf of his fellow.
Who is obligated in the kalbon and who is exempt:
The mishnah states: "Ve'eilu shechayavin bakalbon" - these are the people upon whom the obligation of the kalbon falls:
Levi'im - when a Levi gives a half-shekel. According to the first Tanna, as explained, this applies only when two half-shekels are joined together.
Yisraelim - an ordinary Israelite, and likewise converts and freed slaves.
Kohanim - exempt. Even though it was explained that they are obligated in the giving itself, they were treated leniently and exempted from the responsibility of the kalbon.
Women, slaves and minors - exempt. Since they are not obligated in the half-shekel, even if they did in fact give, they are not required to add the kalbon, for they do not bear the primary obligation.
One who pays on behalf of another:
The mishnah continues: "Hashokel al yad kohen, al yad ishah, al yad eved, al yad katan - patur". One who gives a half-shekel on behalf of a kohen, a woman, a slave or a minor also does not need to add a kalbon. According to the first Tanna, this refers to a case where a person gives a whole coin, and not a half, for himself and for one of these four. Under ordinary circumstances he would be obligated in a kalbon, but here he is exempt, because the other party is not obligated in a kalbon. We view the giving as though he gave for himself alone, and according to the first Tanna one who gives for himself alone does not add a kalbon.
"Shakal al yado ve'al yad chavero":
By contrast, if he gave a shekel for himself and for his fellow - that is, he gave a whole coin instead of a half-shekel - he is "chayav bekalbon echad" - obligated in one kalbon. The reason, as explained, is to cover the money-changer's fee, that payment he would have had to make to the money-changer in order to arrive at the half-shekel, which is his essential obligation.
Rabbi Meir, however, says: "Shnei kalbonot" - two kalbonot. One who gives a whole coin, two halves together, is obligated in two kalbonot, for in his view each half-shekel requires a kalbon of its own, and consequently two half-shekels require two kalbonot.
"Hanoten sela venotel shekel":
With this halachah the mishnah concludes, and it holds true even according to the first Tanna. One who gives a sela - a full sum - and takes back change of a shekel, which is half a sela, it turns out that this shekel is what we call the half-shekel of silver, since half the shekel of the Torah is half a sela. In the era of the Mishnah, the coin that was half of the shekel of the Torah, that is, half a sela, was called a "shekel."
It follows that one who gives a sela and takes back a shekel is "chayav shnei kalbonot" - obligated in two kalbonot, and this is so even according to the first Tanna, because two exchanges take place here: he gives a coin and he also receives a coin back. For this reason we regard it as two exchanges and two payments incumbent upon him, and therefore he is obligated in two kalbonot according to both views.
In summary: in this mishnah we learned about the kalbon, the small addition that sometimes accompanied the half-shekel, and about the dispute concerning its reason: according to Rabbi Meir the kalbon comes to complete the value of the half-shekel, and therefore every half-shekel requires a kalbon; while according to the first Tanna it comes in place of the money-changer's fee, and therefore it is required only when a whole coin is given on behalf of two people. From this the details of the halachah are derived: Levi'im, Israelites, converts and freed slaves are obligated; kohanim, women, slaves and minors are exempt; one who pays for himself and for one of those who are exempt is exempt; one who pays for himself and for his fellow is obligated in one kalbon according to the first Tanna and in two according to Rabbi Meir; and one who gives a sela and takes back a shekel is obligated in two kalbonot according to all opinions, because of the two exchanges he has made.