Shaykh al-Islam Ibn Taymiyyah said, “The Prophet ﷺ affirmed for the caravan-riders the option to rescind when they are intercepted, because the practice involves a kind of concealment and fraud.”

Ibn al-Qayyim added, “He forbade it because it misleads the seller: unaware of the prevailing price, he lets the buyer take the goods for less than their value. For that reason the Prophet ﷺ granted him the option once he enters the marketplace. There is no dispute that this option applies whenever there is substantial overreaching; if the one bringing the goods does not know the going rate, he is ignorant of the normal price and the buyer is thereby deceiving him. The same ruling holds if the seller sells something to those riders: when they come down to the market and realize that they have been underpaid by a margin beyond what people usually tolerate, they too are given the option.” End quote.

The second scenario that gives rise to the option due to overreaching is when the overcharge results from najsh—artificially bidding up the price. A najish is someone who raises the bid on an item while having no intention of buying it, but only to inflate the price for the purchaser. This practice is forbidden, for the Prophet ﷺ said, “Do not practise najsh,” because it deceives and misleads the buyer; it is tantamount to cheating.

One form of the prohibited najsh is for the seller to claim, “I was offered such-and-such for it,” or, “I bought it for such-and-such,” when he is lying.

Another example is for the seller to say, “I will not sell it for less than such-and-such,” merely to make the buyer pay close to that amount—as when an item worth five is offered for ten so that the buyer ends up paying somewhere near ten.