הלכה על שקלים 2:9

From the laws of the commandment is for example, that which they, may their memory be blessed, said (Mishnah Shekalim 1:1) that on the 1st of Adar, we announce about the shekel-coins; that even the poorest of the poor is liable for it, and if he does not have it, he borrows from others or sells the cloak that is upon him and gives it, as it is stated (Exodus 30:16), "and the poor may not lessen." And [that] he does not give [it] over several times, but rather all at one time; that its weight is [equal to] eighty grains of barley, as the weight of the whole shekel that was in the days of Moshe was one hundred and sixty barley [grains]. And that all are obligated to give it, priests, Levites, Israelites, converts and freed slaves; but not women, slaves and minors. But if they give it, we accept [it] from them, but not from gentiles - they do not have a share and inheritance among us. And [that] they, may their memory be blessed, also said (Mishnah Shekalim 1:6) that anyone who does not give an exact half-shekel which was a coin at that time; but gives silver of its weight for it or [its equivalent] in small bronze coins (perutot) [instead], must add a little to the weight of his shekel - and that little was called a kalbon (small coin). And that this little is the wage of the moneychanger who gets his wage for changing the perutot into the half shekel that was fixed. And therefore two that brought a whole shekel between the two of them [also] require a kalbon - since if they wanted to exchange it, they would need to give the moneychanger a kalbon. And so too must they give it to the [Temple] treasurer, since Scripture obligated them in a half shekel (and not a whole one) - and so they are obligated about it or its value precisely. And so [too,] that which they, may their memory be blessed, said (Mishnah Shekalim 2:1) is the law about one who loses his shekel on the way. And the rest of its many details are [all] elucidated in the Tractate built upon this, and that is Tractate Shekalim (see Mishneh Torah, Laws of Sheqel Dues 1).