Our Mishnah deals with a case in which beis din sold property from an estate, and afterward it became clear that the sale was based on a mistaken calculation and for an incorrect sum. What is the status of such a sale?
The Appraisal of the Judges:
We are speaking of an appraisal made by the judges on behalf of beis din, who sell the property of an estate in order to pay a widow her kesubah, or in order to repay a creditor the money the estate owes him.
"Pichasu shtus o hosiru shtus - michran batel" - if they erred in their evaluation and sold the property for a sixth less than its value or for a sixth more than its value, the sale is null and void on account of the overreaching, whether it was to the detriment of the seller or of the buyer.
If they erred by less than a sixth - the sale is not annulled. Unlike the widow, about whom we learned in the previous Mishnah that she has no power to sell beyond that amount, here we are lenient with beis din and place it on a higher level because of its standing.
The View of Rabban Shimon ben Gamliel:
Rabban Shimon ben Gamliel says "michran kayam" - the sale stands even when the error was a sixth. He takes a broader view and argues: "Im kein mah koach beis din yafeh" - if we invalidate their actions for every error, in what way is the power of beis din great, and in what way is it superior? It is fitting that beis din stand on a higher level and be treated with respect even when its appraisal is not one hundred percent accurate, and even when the error amounts to a sixth.
"Aval im asu igeres bikores":
An igeres bikores is a document drawn up on the basis of a public announcement: beis din publicized the sale openly, and from this it is evident that every opportunity was taken to obtain the best value. In such a case, even if they sold something worth one hundred for two hundred, or something worth two hundred for one hundred - an error amounting to half the value - "michran kayam", and the sale is valid even according to the first Tanna. Since an announcement was made and an effort was made to obtain the best possible deal, there is no overreaching here.
In summary: we learned that when the judges' appraisal was mistaken by a sixth, whether less or more, their sale is void on account of overreaching, whereas when the error is less than a sixth the sale stands because of the standing of beis din. Rabban Shimon ben Gamliel upholds the sale even with an error of a sixth, for the reason of "im kein mah koach beis din yafeh." And when an igeres bikores was drawn up, on the strength of a public announcement, the sale stands even with an error amounting to half the value.