Bava Metzia, Chapter 4, Mishnah 7. In this mishnah, the Mishnah begins to deviate slightly from the topic we have been dealing with so far. The first expression is not a new ruling: "Ha'ona'ah arba'ah kesef" - the measure of overcharging is four silver coins, four ma'ot for every sela. As we have already learned, there are twenty-four ma'ot in a sela, and it turns out that four ma'ot are one-sixth of the sela (shtut).
The point here is that since we are dealing with halachic measures that are measured using the small silver coins - the ma'ah - the Mishnah brings, in addition to the measure of overcharging relative to a sela, an entirely new law. It is not connected to our current discussion at all, but rather belongs to a different area in halachah: the law of modeh bemiktzat, and the oath a person takes when he admits to part of a claim.
The law of modeh bemiktzat:
Reuven claims that Shimon owes him money. If Shimon denies it completely, and neither of them has proof, the discussion ends here. But if Shimon admits partially and says, "I do not owe you a hundred, but I owe you forty" - this is a partial admission called modeh bemiktzat. Although he does not pay the balance, the Torah requires him to swear that he does not owe the rest. The underlying assumption is that one who admits partially is likely trying to buy time: he is incapable of admitting to his entire debt, even though in truth he owes the whole amount.
What, then, is the measure of the claim that has the power to obligate this oath? We have two separate parameters before us: the size of the total claim, and the size of the admission.
"Vehata'anah shtei kesef" - the total claim must be in the measure of two silver coins, meaning two ma'ot.
"Vehahoda'ah shaveh perutah" - what he admits to must be worth at least a perutah. The perutah is the smallest of the copper coins, just as the ma'ah is the smallest of the silver coins, and it is a tiny coin. He must admit to something worth at least a perutah so that his admission is considered a partial admission that obligates an oath.
A ma'ah is worth thirty-two perutot, so it turns out that two ma'ot are sixty-four perutot. According to the explanation before us - as the Bartenura explained, and this is the opinion of Shmuel in the Gemara - the total claim is sixty-four perutot, the admission is one perutah, and the denial is regarding the remaining sixty-three. On these he swears and is exempt. With less than this, whether in the total claim or in the admission, the Torah obligation for an oath does not begin at all.
This is a simple way of learning it, but the halachah follows the opinion of Rav, and so the Rambam and the Shulchan Aruch rule: the part that he denies must be at least two ma'ot. That is, if he admitted to one perutah, the original claim was sixty-five perutot: an admission to one, and a denial of sixty-four, which equal two ma'ot. It turns out that a denial of two ma'ot and an admission worth a perutah are required for the obligation of the modeh bemiktzat oath to take effect.
From here onward, the Mishnah moves slightly away from the topic: incidentally to mentioning the measure of a perutah in the law of modeh bemiktzat, it lists all the areas in halachah where the perutah is the determining measure, five in number.
The five cases where the measure is a perutah:
Modeh bemiktzat - the law that was just explained: the defendant must admit to at least the value of a perutah in order to become obligated in the modeh bemiktzat oath.
"Veha'ishah mitkadeshet beshaveh perutah" - one who gives a woman money for the sake of kiddushin can betroth her with money or items of monetary value. Today, the custom is to betroth with a ring, but the value of the ring is what is actually given. Therefore, what is given must be worth at least a perutah, the smallest coin.
"Vehaneheneh beshaveh perutah min hahekdesh ma'al" - the law of me'ilah, which is the unlawful use of consecrated property. From the moment sanctity takes effect upon an object - whether it is bedek habayit consecration, which is only for its monetary value and the treasurer will eventually sell it, or whether it is altar consecration that goes up onto the altar itself - it is forbidden to use it for personal benefit. If he used it inadvertently, meaning unintentionally, this is me'ilah, and it has two consequences: bringing an asham me'ilot offering, which is a ram, and returning the value of the item he unlawfully took plus a fifth. The measure for the obligation is that the benefit must be worth a perutah: "haneheneh beshaveh perutah min hahekdesh" - "ma'al", and he transgressed the prohibition of me'ilah.
"Vehamotzei shaveh perutah chayav lehachriz" - this is the novel ruling of our mishnah, since the other four cases have already been mentioned in other places in the Talmud. The verse specifies that the obligation to return is regarding a thing "that is lost from him", and therefore the object must reach the threshold that defines it as a lost item so that the obligation to announce it and return it will take effect. If someone lost an insignificant item that is not worth even a perutah - including a single pin that fell, even if his name is engraved on it and it therefore has an identifying mark through which he could receive it back - this is not a lost item, because a person does not bother to pick up such a thing from the floor due to its meager value. Therefore, there is no obligation to announce it, and it is permitted to ignore it, because it is not worth even a single perutah.
"Vehagozel et chaveiro shaveh perutah venishba lo, yolichenu acharav afilu leMadai" - this is the special case of a false oath. Reuven claims that Shimon owes him money, for whatever reason: he lent him money, robbed his wallet, found his wallet, received a wallet from him to guard as a deposit, or owes him wages. In any of these cases, if the defendant swore in court that he does not owe him, and later admitted that he lied in his oath, and now he seeks to atone and rectify what he distorted - a special law is stated regarding him.
In a regular robbery, the mitzvah is "and he shall return the stolen item that he robbed", and there the robber is permitted to demand that the victim come to him, or to send him the money via a messenger. However, here, once he swore falsely regarding a debt that he owed, he is obligated to deliver personally and individually all that he owed. Even if the victim moved to a distant place - to Media - the robber must go all the way in order to hand the money to him directly. We learned this in Tractate Bava Kamma, Chapter 9, Mishnah 5.
Therefore, our mishnah states: "Hagozel et chaveiro shaveh perutah" - he robbed him of an item worth at least a perutah, "venishba lo" - and he swore to him falsely that he does not owe him, and as stated, this does not apply only to robbery but to any way in which he owes him money, and afterward he admitted - "yolichenu acharav afilu leMadai", and he must go all the way to the location of the creditor anywhere in the world, even as far as Media.
In summary: In this Mishnah we learned the measure of ona'ah - four kesef out of twenty-four ma'os in a sela; the law of modeh bemiktzas and its measures - a claim of two kesef and an admission of a perutah's worth, and the halachah in accordance with the opinion of Rav that the denial itself must be two ma'os; and the five cases where a perutah is the determining measure: partial admission, betrothing a woman, me'ilah with hekdesh, the obligation to announce a lost object, and bringing stolen property after the person who was robbed following a false oath. In all of these, for less than a perutah's worth the law does not apply - and this includes one who swears falsely over less than a perutah's worth, who is not required to bring the money after his fellow even to Media.