Normal inflation, for example. Here we'll talk about a situation where one person gives another person his food to be the shomer, to safeguard that food, and then the food starts to decrease. It starts to get moldy, or mice start eating from it, and now the shomer has that food in his possession. Should he sell it and retain the value, or should he hold on to the actual food, even though its value is constantly decreasing because mice are eating it, for example?
The Mishnah says, “Even if he sees that the food is slowly becoming ruined, the shomer shouldn't touch it; he's not allowed to sell it.” Even though he could save money for the person who deposited the food with him, he should not sell it. The Gemara comments: “A person would rather have his one kav—a kav is a measurement—of his own produce than nine kav of produce from his friend, from somewhere else.” The reason is that a person loves the produce he works for very much and appreciates his own produce because he worked hard for it, rather than nine kav of produce from somebody else who worked for it. Somebody enjoys the fruits of their own labor.
Therefore, even though the amount of food decreases because of the mice, the shomer shouldn't sell it; he should hold on to it because the person wants his own stuff. But here we're talking about a regular decrease, a normal decrease that happens in the market. It normally goes down a few percent a year in value. But the Gemara comments here and says, Aval im Avdu, if there's an abnormal amount of loss, then it becomes an issue, and in that case, he would sell it for his friend.
Rabban Shimon ben Gamliel disagrees. Rabban Shimon ben Gamliel omer—says Rashbag—the shomer sells it in front of the Beit Din (rabbinical court). He sells it once the Beit Din has given him permission. In this case, it's like a person returning a lost object to its owner. According to Rashbag, this Tanna (early rabbinic sage) says the owner wants his produce to be sold. He doesn't want to have further loss, even though he's now going to replace it with the produce of others, because he can take the money his own produce was sold for and buy other produce.
Therefore, selling the produce is considered a benefit for him, as is returning his own property. So Rabban Shimon ben Gamliel says the shomer should sell the produce, retain that amount of money, and eventually return the money to the owner, who can buy new produce with it. But we should note that this is only in a situation where the owner is not in town and you can't clarify what he wants done with his own food. If he is in town, then the shomer is not allowed to sell him. He has to go to the owner and inform him of what's going on before the owner decides what to do.