Beginning with the Mishna on daf 49b, the rest of the perek focuses on issues of ona’ah. Ona’ah is a biblical prohibition (see Vayikra 25:14), which forbids someone to take unfair advantage of another by overcharging or undercharging when negotiating a business deal. There are three different levels of ona’ah established by the Sages:
1. When the ona’ah – the amount that is overcharged – must be returned
2. When the ona’ah is so small that we assume the parties do not care about it, and it does not have to be returned
3. When the ona’ah is so large that the entire transaction is nullified.
The amount that is set by the Mishna as ona’ah is one-sixth of the value of the transaction. Therefore, if the overcharge is exactly one-sixth, that money is returned; if it is more than one-sixth the transaction is nullified; if it is less than one-sixth the transaction stands and the money need not be returned. The Mishna teaches that there is a time limit on demanding a return of the ona’ah – the amount of time that the purchaser needs to show the object to a knowledgeable friend or businessman. If he holds it for longer than that period, we assume that he has accepted the price, and that there is mehila – that he “forgives” (waives) any money that is owed to him.
In discussing mehila in the case where the amount of the overcharge is less than one-sixth, the rishonim disagree as to whether there is any prohibition at all. According to the Sefer HaHinnukh, since everyone “forgives” such a small amount, the seller did nothing wrong. The Ramban disagrees, arguing that it is forbidden even if the money need not be returned. In explaining the Ramban’s position, the Minhat Hinnukh says that really the money should be returned, but because of the idea of mehila we do not obligate people to do so.