According to the Mishna (93b) it is reasonable to anticipate that there will be a certain amount of waste with produce that is purchased. Ten figs out of every hundred, ten barrels of slightly sour wine out of every hundred, ten bad jugs out of every hundred, and up to a quarter kav of impurities per se’a of grain. Rabbeinu Gershom explains that this is the normal amount of waste that can be expected during the harvest season, given that other plants also grow in the fields together with the grain.
On our daf, the Gemara brings Rav Huna’s teaching that a buyer can sift the grain that was purchased, and if he finds that there is more than a quarter kav of waste he can demand that the seller replace the entire amount that he had agreed to sell, not only the three-quarters that was to be expected. Two approaches are offered in explanation of Rav Huna’s ruling – either it is dina, this is what is owed according to the letter of the law, or it is kenasa – it is a penalty.
The Gemara explains:
If we view this as a the letter of the law, then we assume that the person who makes such a purchase really intends to purchase the full amount, and that ordinarily he is not concerned with a certain amount of waste. Since this individual showed that he did care about the waste, he must receive the full amount.
If we view this as a penalty, then we are saying that a quarter kav can be expected. Since in this case there was more than the average amount, we suspect that the seller had added in “filler.” Now that it has been discovered, he will have to pay the full amount.