There is a a law in the Rambam that does not seem to ''shtim'' [be in accord] with the Gemara in Ketuboth 76b. The Gemara brings a case where a cow was exchanged with an ass, and the cow was present, but the ass was still in the barn of its owner. When the owner of the cow went to pick up the ass, it was found dead. Rav Yehuda said in the name of Shumel, the owner of the ass has to bring proof that it was alive at the time of the exchange. Mari bar Yechezkeil said ''Pay no attention to what Yehuda said. Rather in whose domain a doubt arose, upon him is to bring a proof.'' The Rambam writes in laws of sales 20 law 14 in whose domain a doubt arose, upon him is to bring proof. For example a cow was exchanged with an ass. When the owner of the cow went to pick up the ass, it was found dead. The owner of the ass has to bring proof that it was alive at the time of the exchange. Here the Rambam brings the statement of Mari who disagrees with Rav Yehuda and right away the statement of Rav Yehuda. To make my question clearer, let me make clear that the Rambam is saying even though the owner of the cow now owns the ass, still since the ass is in the barn of the [previous] owner of the ass, the previous owner must bring proof. While Mari is saying that the owner of the cow has to bring proof. Besides this, the meaning of Mari is when he says ''domain'' means ownership, not in the property or on the land of. Otherwise he would be saying the exact same thing as Rav Yehuda with whom he is disagreeing.
this is clearly very relevant to bava mezia page 100a but i do not recall that sugia. i recall writing about it in my little booklet on bava metzia chapters 8 and 9 but i have no recollection of what i wrote. while i am at it i might mention that this idea here i jut put into a different book on shas
----------------------------------------------------------------------------------------------------------------
I am confused about a רמב''ם that does not seem to be in accord with the גמרא in כתובות ע''ו ע''ב. The גמרא brings a case where there a cow was exchanged with an חמור, and the cow was present, but the חמור was still in the barn of its owner. When the owner of the cow went to pick up the חמור, it was found dead. רב יהודה said in the name of שמואל, the owner of the חמור has to bring proof that it was alive at the time of the exchange. מרי בר יחזקאל said ''Pay no attention to what יהודה אחי said. Rather in whose domain a doubt arose, upon him is to bring a proof.'' The רמב''ם writes in בהלכות מכירה כ' הלכה י''ד in whose domain a doubt arose, upon him is to bring proof. For example a cow was exchanged with an חמור. When the owner of the cow went to pick up the חמור, it was found dead. The owner of the חמור has to bring proof that it was alive at the time of the exchange. Here the רמב''ם brings the statement of מרי who disagrees with רב יהודה and right away the statement of רב יהודה. To make my question clearer, let me make clear that the רמב''ם is saying even though the owner of the cow now owns the חמור, still since the חמור is in the barn of the [previous] owner of the חמור, the previous owner must bring proof. While מרי is saying that the owner of the cow has to bring proof. Besides this, the meaning of מרי is when he says ''רשות'' means ownership, not in the property or on the land of. Otherwise he would be saying the exact same thing as רב יהודה with whom he is disagreeing.
הרמב''ם לא נראה כמתאים לגמרא בכתובות ע''ו ע''ב. הגמרא מביאה מקרה
שבו הוחלפה פרה עם חמור, והפרה הייתה נוכחת, אבל החמור עדיין היה ברפת של בעליה.
כשבעל הפרה הלך לקחת את החמור, היא נמצאה מתה. אמר רב יהודה בשם שמואל, בעל החמור
צריך להביא הוכחה שהוא היה חי בזמן ההחלפה. מרי בר יחזקאל אמר ''אל תשים לב למה
שאמר יהודה אחי. אלא שבתחומו התעורר ספק, עליו להביא הוכחה.'' הרמב''ם כותב בהלכות
מכירה כ' הלכה י''ד של מי שבתחומו התעורר ספק, עליו להביא הוכחה. למשל פרה הוחלפה
עם חמור. כשבעל הפרה הלך לקחת את החמור, היא נמצאה מתה. בעל החמור צריך להביא
הוכחה שהוא היה בחיים בזמן ההחלפה. כאן מביא הרמב''ם את דברי מרי החולק על רב
יהודה ומיד את דברי רב יהודה. כדי להבהיר את שאלתי, הרשו לי להבהיר שהרמב"ם
אומר אף על פי שבעל הפרה הוא עכשיו בעל החמור, עדיין כיון שהחמור נמצא ברפת של
הבעלים [הקודמים] של החמור, הבעלים [הקודמים] חייבים להביא הוכחה. ואילו מרי אומר
שבעל הפרה צריך להביא הוכחה. חוץ מזה, הכוונה של מרי היא כשהוא אומר ''רשות''
פירושו בעלות, לא בקרקע של. אחרת הוא היה אומר בדיוק אותו דבר כמו רב יהודה שהוא
לא מסכים איתו
