Gittin, Chapter 5, Mishnah 3. This mishnah teaches that for the sake of tikkun ha'olam - the good order of society - the Sages enacted that there are cases in which a person is entitled to certain payments, and nevertheless he cannot collect them from liened property, that is, from property that has already been sold, but only from unencumbered property: property currently in the hands of the debtor himself.
The text of the mishnah:
"Ein motzi'in la'achilas peiros" - we do not collect from the debtor for the consumption of produce.
"Lishvach karka'os" - and likewise for the improvement of land.
"Limzon ha'ishah v'habanos" - and likewise for the maintenance of the wife, and in this case it refers to his widow, and the maintenance of the daughters after his passing.
"Minechasim hameshubadim" - from property that has been sold.
"Mipnei tikkun olam" - this is the reason for the enactment, as will be explained.
The case of consumption of produce and improvement of land:
Reuven stole a field from Levi and sold it to Shimon. Once Levi took his field back from Shimon, Shimon came to Reuven to collect his loss.
The value of the land: the actual purchase price of the field Shimon may collect even from liened property, that is, from fields that Reuven the thief sold to other people.
Consumption of produce and improvement of land: when Levi took the field back, he took along with it the produce that grew in that land in which Shimon had labored, as well as the other improvements he had made in it. For these items Shimon does not collect from Reuven's liened property, but only from unencumbered property - actual assets that Reuven, the seller, currently holds in his possession and has not sold to anyone else.
Maintenance of the wife and daughters:
By virtue of the terms of the kesubah, a husband is obligated to support his wife, and after his passing his widow continues to be maintained from his property (and it appears that the law applies even during his lifetime). So too the daughters, whose maintenance is part of the terms of the kesubah, are maintained after his passing from the estate. These funds as well are collected only from unencumbered property, and not from liened property that has already been sold.
The reason for the enactment:
We cannot expect a person purchasing property to be preoccupied with searching for bottomless pits. When a person buys a parcel of land, he understands that there is a certain degree of risk - it is possible that the property will be taken back from him, and that he will have to sue in court the one who sold it to him (this is in fact the reason that nowadays people buy title insurance). This risk is known and can be assessed in advance. But these three items - consumption of produce, improvement of land, and maintenance of the wife and daughters - are a sort of open-ended obligation, and there is no way to assess in advance how much they will accumulate to, and therefore we cannot expect that they be collected from liened property.
"Hamotzei metziah lo yishava":
The last item in the mishnah deals with someone who found a lost object and returned it to its owner. The owner cannot obligate him to swear that he returned the entire lost object. Even if the owner claims, "You are not returning everything to me, I know that part of it is missing," he cannot demand that the finder swear that he returned the item in its entirety.
And the reason, again, is for the sake of tikkun ha'olam: if people knew that they might be subjected to an oath, they would ignore lost objects and say, "Why should I get myself involved in this?" Therefore, so that people will return lost objects, the Sages established that the owner may not demand of the finder an oath confirming that he returned the entire item.
In summary: in this mishnah we learned that for the sake of tikkun ha'olam we do not collect for consumption of produce, improvement of land, and maintenance of the wife and daughters from liened property, but only from unencumbered property, because these are obligations whose measure cannot be assessed in advance and cannot be imposed upon buyers; and likewise that the finder of a lost object does not swear to the owner, so that people will not refrain from returning lost objects.