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.