Bava Metzia 47A

ויש לו עליו אונאה.

And if the sum of money is 1/6th lower than the value of the item, the seller of the item has a claim of exploitation against the buyer, who must pay the difference to the seller.

קנה ⁠– אף על גב דלא משך, דכיון דלא קפיד ⁠– קנה, דכי חליפין דמי. ויש לו עליו אונאה ⁠– ד״מכור לי באלו״ קאמר ליה. רב אבא אמר רב הונא: ״מכור לי באלו״⁠ – קנה, ואין לו עליו אונאה.

The Talmud elaborates: The buyer acquires the item even though he did not pull it, because since the seller is not particular about the sum, the buyer acquired the item, as it is similar to a transaction effected by means of exchange. And the seller has a claim of exploitation against the buyer because the buyer said to him: Sell me your item for these coins. The use of the language of sale indicates that it will be for an acceptable price. Rav Abba says that Rav Huna says that if the buyer said: Sell me your item for these coins, the buyer acquired the item; and the seller of the item has no claim of exploitation against the buyer, as it is a full-fledged transaction effected by means of exchange.

פשיטא: דמים ואין מקפיד עליהן ⁠– הא קאמרינן דקני, דכחליפין דמו. חליפין ומקפיד עליהן, מאי?

The Talmud analyzes these halakhot: It is obvious that in a case where there is a sale of an item for money and the seller is not particular about the sum, we say that the buyer acquires the item immediately upon receiving the money, as it is similar to a transaction effected by means of exchange. But if it is a transaction effected by means of exchange and one of the parties is particular that the value of the items is equal, what is the halakha? Is its legal status that of a sale, or that of a transaction effected by means of exchange?