Why doesn't the Gemara ask a contradiction from מתנה? (Bava Metzia 19a)

My chavrusa asked this morning why doesn't the Gemara ask a contradiction between the ברייתא and the Mishna by the case of מתנות? The Mishna says מתנה as one of the cases and the ברייתא says the case of מתנות as well. By מתנה you can't answer like the Gemara did for דייתיקי.

The Rishonim seem to have 2 approaches to answer the question.

1. There is a מחלוקת Rashi and Tosafos what is pshat in אביי when he explains what the ברייתא means by מתנה that it is מהיום ולאחר מיתה. Rashi understands that he is מקנה the גוף of the קרקע now and he retains the פירות until his death. Because he gave away the גוף of the קרקע now, he can not back out from this מתנה as opposed to a מתנת שכיב מרע, where he can always back out. Tosafos asks on Rashi, if he can't back out how can the gemara compare it to a מתנת ?שכיב מרע Therefore Tosafos says that מהיום ולאחר מיתה includes a תנאי that he can back out until he dies.

Based on this the Tosafos Harosh and others answer the question as follows. They explain that מתנה in the Mishna is like אביי, a case of מהיום ולאחר מיתה and he can back out (לשיטתם). Therefore there is no contradiction between the Mishna and the ברייתא as ר' אבא בר ממל answered. The Mishna (even by מתנה) is where he can be חוזר and therefore a שטר with an earlier date doesn't help him while the ברייתא is talking about a regular מתנה where he cannot be חוזר and therefore even if he says תנו אין נותנים because we are afraid that he never gave him the שטר and in the meantime he gave it away to someone else. Therefore if we give back the שטר we are going to be מפסיד the second guy.

2. Rashi says (י"ט: ד"ה קמא זכה at the end) that we only make the דיוק of אם אמר תנו נותנים where the reason of the Mishna doesn't apply, namely by a שכיב מרע since because he can back out it doesn't matter whether he gave the שטר or not. However, by מתנה where the reason of שאני אומר that he didn't give the שטר applies even if he said תנו, even of he said תנו we are not מחזיר.

This is difficult for a number of reasons. The Gemara doesn't make this distinction. The Gemara seems to say that the דיוק of אמר תנו נותנים applies to all the cases. If it applies to everything but מתנה (which it does) why does the gemara list מתנה with the other cases? The דיוק also seems to be based on the language of the Mishna, the same language is used by מתנה as the other cases.

Is there מחזי כריבית by פירות? (Bava Metzia 14b)

Shmuel only said explicitly מחזי כריבית by שבח, what about by פירות? This is aמח' ראשונים and affects how we understand the Gemara's question from the ברייתא of לאכילת פירות כיצד.

The רשב"א says that just like by שבח there is מחזי כריבית there is מחזי כריבית by פירות as well because he didn't actually buy anything. Therefore, the question from the ברייתא of לאכילת פירות כיצד is as was explained in the shiur.

However the תוס' הרא"ש says that by פירות there is no מחזי כריבית because by פירות the פירות are a tangible asset and it is clear that he is being paid for them. Only by שבח it is מחזי כריבית because the שבח is intangible and therefore looks like ריבית. Because of this (and other difficulties עיי"ש) the question from the ברייתא can't be a question on ר' נחמן as Shmuel would agree by פירות that the לוקח can collect the money because it is not מחזי כריבית. Therefore, he has a completely different pshat in the Gemara's question.

The Gemara is asking as follows. The Gemara thinks that the ברייתא is a proof to ר' נחמן, because the ברייתא has 2 distinct cases פירות and שבח. If a לוקח מגזלן got שבח then the ברייתא should have combined פירות and שבח into 1 case as they are the same דין. The fact that it split it into 2 cases is a proof that they are 2 distinct cases (בע"ח and גזלן). It turns out that the Gemara's question is actually on רבא, what is the chiddush of the 2 cases if they are both by גזלן, it should be the same din that he gets the money and the ברייתא should have combined them into 1 case. רבא answers with a different אוקימתא and ultimately, the chiddush is that even though there was a גזר דין on the קרן we don't say there was a קול on the פירות.

אין שליח לדבר עבירה (Bava Metzia 10b)

There is a מח' ראשונים ואחרונים what is the reason for אין שליח לדבר עבירה. The תוס' הרא"ש here ד"ה הני as well as the סמ"ע in סי' קפ"ב ס"ק ב hold that the reason is a סברה because the משלח thinks that the שליח won't do the שליחות because דברי הרב דברי התלמיד דברי מי שומעים. However if he knows that the שליח will do the עבירה he is חייב because the סברא does not apply. The סמ"ע seems to hold that אין שליח לדבר עבירה is a problem in the appointment of the שליח. If I am not confident that he will do the שליחות then I am not really appointing him and he is not my שליח.

The ריטב"א in קידושין מ"ב and רעק"א here both say that it is a גזירת הכתוב and not a סברה. The ריטב"א says that the Gemara mentioned the סברה because it applies most of the time but even where the סברה does not apply, for example שוגג (see below), we say אין שליח לדבר עבירה in any case.

תוספות in a number of places argue with the ריטב"א and say that יש שליח לדבר עבירה where the שליח is a שוגג. This would seem to be like the סמ"ע, that since the שליח doesn't know it is אסור the משלח is confident that he will do the שליחות and therefore it is a good appointment. Similarly, the רמ"א writes (סי' שפ"ח סע' ט"ו) that if the שליח is הוחזק to do this עבירה you don't say אין שליח לדבר עבירה and the משלח is חייב. The reason would seem to be like the סמ"ע, that since he is הוחזק you believe he will do the עבירה and therefore you really are appointing him.

The gemara asks that an עבד ואשה since they don't have to pay we should say יש שליח לדבר עבירה and the Gemara answers that since if they get divorced or freed they have to pay that is called a בר חיובא. The Gemara's ה"א is very difficult. רעק"א and the רש"ש ask even if they don't have to pay they are certainly עובר the איסור of גזילה so how can the Gemara possibly think that they are called a לאו בר חיובא just because they won't have to pay back the money? Since they are violating the איסור we should still say אין שליח לדבר עבירה

The question would seem to be לשיטתו that אין שליח לדבר עבירה is a גזירת הכתוב and not a סברה however, according to the סמ"ע we can answer as follows. The ה"א was that since they won't have to pay, the משלח thinks that they will do the עבירה and therefore the מינוי is good. The Gemara answers that since they may eventually have to pay, the משלח is not confident that they will do it and therefore we would say אין שליח לדבר עבירה. The ה"א is very similar to where the שליח is a שוגג, since you think he will do the שליחות we say יש שליח לדבר עבירה