הלכות מכירה

When a person exchanges one article for another, or one animal for another, the laws of ona'ah do not apply. This is true even when he exchanges a needle for a necklace, or a lamb for a donkey. This person may desire the needle more than the necklace.
When, however, a person exchanges produce for produce, the laws of ona'ah do apply, regardless of whether the produce is evaluated before the sale or after the sale.

When a person purchases an article with coins without measuring their number, volume or weight - e.g., he took a handful of coins and said: "Sell me your cow for these" - this is considered an exchange. The transaction is completed, and the unfair gain must be returned, as explained above.
This law also applies if someone purchases fruit without measuring its number, volume or weight for a sela or two. The transaction is completed and the unfair gain must be returned. When a homeowner sells his personal belongings, the laws of ona'ah do not apply. For a man would not sell his personal belongings unless he were offered an exorbitant amount of money.

Although a person tells a colleague, "We are completing this transaction on the condition that you do not hold me responsible for the unfair gain," the laws of ona'ah apply.
When does the above apply? When the statements are made without being explicit. In such an instance, the other party does not know how much money he is forgoing in favor of his colleague. Needless to say, this is the law when one tells the other: "We are completing this transaction on the condition that there is no ona'ah involved," for ona'ah is involved.
If, however, one explicitly mentions the amount of unfair gain, the laws of ona'ah do not apply, because all conditions that are accepted by both parties are binding in cases of financial law.

What is implied? If the seller tells the purchaser: "I am selling you this article for 200 zuz although I know it is worth only 100. I am selling it to you on the condition that you do not hold me responsible for the unfair gain," the laws of ona'ah do not apply.
Similarly, if the purchaser tells the seller: "I am purchasing this article from you for a maneh although I know it is worth 200. I am purchasing it from you on the condition that you do not hold me responsible for the unfair gain," the laws of ona'ah do not apply.

When a person buys and sells in a faithful manner, the laws of ona'ah do not apply.
What is implied? The seller tells the purchaser: "I purchased this article for such and such, and I am making this and this amount of profit," the laws of ona'ah do not apply.

When a person buys and sells in a faithful manner and purchases many different utensils and garments in one lot, he should not calculate and sell the inferior items in a faithful manner, and the superior items according to their value. Instead, he should sell either the entire lot in a faithful manner, or the entire lot according to its value.
The seller may include the cost of porters, donkey drivers and his lodgings in the cost of the article. He may not, however, add his own wages as a worker to the cost. Instead, he must say: "I am taking this and this amount as profit," and be specific about that amount.

The laws of ona'ah do not apply with regard to a gentile. This is implied by Leviticus 25:14, which states: "When you sell an entity... or purchase an entity..., one man should not cheat his brother."
If a gentile takes unfair advantage of a Jew, he is required to return the unfair gain according to our laws. Dealings with a gentile should not be more severe than dealings with a fellow Jew.

The laws of ona'ah do not apply with regard to the following objects: landed property, servants, promissory notes and consecrated property. Even if one sells one of these entities that is worth one thousand zuz for one zuz, or an entity worth one zuz for one thousand zuz, the laws of ona'ah do not apply.This is derived from Leviticus 25:14, which states: "Or when you purchase an entity from the hand of your colleague." "From the hand of indicates that the verse is referring to an article whose ownership is transferred from hand to hand, thus excluding landed property. And this also excludes servants, for an association has been established linking the laws governing them to those governing landed property. It also excludes promissory notes, since one is not purchasing the note itself. It merely serves as proof of the obligation it records.
"Your colleague" excludes consecrated property.

When does the above apply? When one sells one's own property. When, by contrast, an agent makes the slightest error with regard to the value of either movable property or landed property, the transaction is nullified.
It appears to me that the same laws apply to a guardian entrusted with the property of orphans. If he makes the slightest error with regard to the value of either movable property or landed property, the transaction is nullified. He is not considered comparable to a court, because he is merely one individual.

The following laws apply when a court sells either movable property or landed property belonging to orphans, and errs. If they erred with regard to less than one sixth of the value of the object, it is considered as if the unfair gain is waived, as is the law with regard to an ordinary person. If they erred with regard to one sixth, the transaction is nullified.
If the court did not desire to nullify the transaction, but instead have the unfair gain returned, they may. For the legal power of an ordinary person should not be greater than that of a court.

It appears to me that if a court sold landed property or servants belonging to orphans at an unfair price, selling entities worth 100 zuz for 200, the purchaser cannot retract. For the legal power of an ordinary individual should not be greater than that of orphans. Similarly, if a guardian of orphans sells landed property or servants at an unfair profit, the purchaser cannot retract because of the unfair profit, as is the law with regard to an ordinary individual.

When brothers or partners divide movable property, they are considered like purchasers. If there is unfair gain less than one sixth of the article's value, the transaction is completed, and there is no need to return anything. If the unfair gain exceeds one sixth, the transaction is nullified. If it is an even sixth, the transaction is completed and the unfair gain must be returned.
If the involved parties made an agreement to divide the property according to the evaluation of judges, and the judges erred with regard to a sixth or more, the division is nullified. For when judges who make an evaluation reduce or increase the value of an article by a sixth, the transaction is nullified.

Livestock, pearls, a sword, and a Torah scroll are considered like other movable property, and the laws of ona'ah apply to transactions involving them. There are no entities in which the laws of ona'ah do not apply, except the four entities mentioned by our Sages. And even with regard to these entities, this is true provided the person is buying or selling his own property, as we have explained.

Just as the laws of ona'ah do not apply to the purchase of landed property, so too, these laws do not apply to the rental of landed property. Even if one rents a large hall for only one dinar a year, or a small coop for a dinar a day, the laws of ona'ah do not apply.

When a person hires a worker to work for him - whether with regard to movable property or with regard to landed property - the laws of ona'ah do not apply. It is as if he is purchasing him for that time, and the laws of ona'ah do not apply with regard to servants.

There is an unresolved doubt in the following situation. A person hires a contractor to sow a field, the contractor claims to have sowed it in a proper manner, and witnesses testify that the contractor sowed fewer seeds than are supposed to be sown. We are unsure whether the laws of ona'ah apply because of the seeds, or whether these laws do not apply because of the land.
Therefore, the funds should not be expropriated from the defendant, nor is he required to take anything but a Rabbinic oath, because of the connection the dispute has to landed property.

The laws of ona'ah apply with regard to the rental of a utensil or an animal. For rental is considered to be a sale for a day. Thus, if there is an unfair gain of one sixth or more - regardless of whether this unfair gain is taken at the advantage of the owner or the person who rents the article - the unfair gain must be returned. This applies even when a lengthy period of time has passed since the transaction.