Resources for Kesubos 100
1. The משנה says that if ב"ד sells the property of the יתומים and they sell it for more or less than a sixth of its value, the sale is בטל. Many of the ראשונים ask the following question: We can understand why if ב"ד sells the property for too low that the sale should be בטל since they hurt the יתומים, and it’s a type of לתקוני שדרתיך ולא לוותוני. However, if ב"ד sold their property for more than the value, why should the sale be cancelled? They helped the יתומים and אין אונאה לקרקעות so the sale should be valid! The רא"ש in סימן ט"ז answers that just as the יתומים rely on ב"ד so does the לוקח so the ב"ד are really acting as the the שליחים of both. The רא"ש adds that that it has to be this way or no one would ever buy from יתומים. The ר"ן on the משנה quotes the רמב"ן who says that our משנה is talking about a specific case where ב"ד tried to sell it at its value and couldn’t so they forced the בעל חוב to accept the land as payment. Therefore, if they over charged him then the מקח is בטל. The ראב"ד in הלכות מכירה פּרק י"ג הל׳ ט says that normally people are מוחל by less than a sixth or by קרקע. However, since no one dreams that ב"ד will make a mistake the לוקח isn’t מוחל. Theרמב"ם in the above mentioned פּרק in הלכה י וי"א says that the answer to the question is אין הכי נמי. In other words, if ב"ד charges too much then the sale is valid since “ב"ד can’t be worse than a הדיוט”. Therefore, if its מטלטלין then it’s a sixth and if its land then there is never אונאה. (I believe the רמב"ם was מדייק this הלכה from the fact that רשב"ג said "מה כח ב"ד יפה” which means he believes ב"ד must be better than a הדיוט so the חכמים wouldn’t go to a far extreme and say they are worse since why widen the מחלוקת”). The רא"ש also brings רב האי גאון who says that even by a regular שליח, if he sells the property too high and it is more than a sixth then the sale is בטל. The רא"ש doesn’t understand this since it should be that the sale is valid if it helped the seller and if he is learning that the seller must be equal to the buyer or no one would buy from a שליח then even if he overcharged by a דינר it should be בטל because the seller could have said לתקוני שדרתיך ולא לעוותוני so the buyer could say it as well. The הפלאה has a beautiful תירוץ. He says that רב האי גאון understood that אונאה of a sixth is an עברה. However, less than a sixth is not an עברה (this is a שאלה discussed in שולחן ערוך סימן רכ"ז). If so, when a שליח overcharges by a sixth he has done an עברה and we would say אין שליח לדבר עברה and the sale would be בטל. However, if it was less than a sixth it’s not an עברה so the שליחות is still valid.
2. The משנה says that a ממאנת doesn’t get a כתובה. רש"י explains that this is because she decided to leave so the husband shouldn’t have to pay for that. תוספות in יבמות דף ס"ה ע"ב ד"ה כי disagrees and says that a ממאנת doesn’t get a כתובה because מיאון uproots the marriage למפרע so it turns out she was never married so there is no חיוב כתובה. Why doesn’t רש"י give that simpler explanation? Many explain that this goes well if with the famous חידושי רבינו חיים הלוי in הלכות אישות פּרק ב הל׳ ט who says that מיאון only uproots the marriage מכאן ולהבא למפרע. Meaning for any הלכה in the future we say it is as if they were never married. However, we don’t change whatever happened already. So if the wife had eaten מזונות till this point she doesn’t need to pay back the husband for them even though the marriage was uprooted retroactively. Similarly, since the חיוב כתובה is a preexisting debt, he still needs to pay it if not for רש"י’s סברא that she decided to end it.