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The Mishnah in Shvuos 27a says:

If one takes an oath to refrain from performing a mitzva and he does not refrain, he is exempt from bringing an offering for an oath on an utterance. If he takes an oath to perform a mitzva and he does not perform it, he is also exempt, though it would have been fitting to claim that he is liable to bring the offering, in accordance with the statement of Rabbi Yehuda ben Beteira.

But the Gemara in Nedarim 8a says you can. What is the answer to this contradiction?

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    This question would be more accessible if you included quotations of the relevant passages.
    – Double AA
    Nov 18 '20 at 18:20
  • See also Yoma 73b–74a. Nov 18 '20 at 20:42
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It's not entirely clear from the gemara in Nedarim that the shevua actually works - the below is the Sefaria translation:

מנין שנשבעין לקיים את המצוה שנאמר (תהילים קיט קו) נשבעתי ואקיימה לשמור משפטי צדקך

From where is it derived that one may take an oath to fulfill a mitzva? It is as it is stated: “I have sworn and I have confirmed it, to observe Your righteous ordinances” (Psalms 119:106).

והלא מושבע ועומד מהר סיני הוא אלא הא קמ''ל דשרי ליה לאיניש לזרוזי נפשיה

The Gemara asks: Is he not already under oath from when each Jew took an oath at Mount Sinai to fulfill all the mitzvot? An oath cannot take effect if one is already bound by a different oath. Rather, it teaches us this: It is permitted for a man to motivate himself to fulfill the mitzvot in this manner...

The Gemara itself asks that the oath should be invalid, because it's an already existing obligation (see commentators that this is based the mishna in Shevuous quoted in the question).

The Ran, as mentioned by @Kouty, explains that the oath is valid, but with no korban shevuah obligation, and struggles to explain the gemara's answer דשרי ליה לאיניש לזרוזי נפשיה (see inside).

The Rosh, (and following him Sefaria who write: "although the oath is not technically valid", though I deleted that bit from the quote) explains that the shevuah doesn't work at all, but it's permitted to make as an encouragement to oneself and is not a unnecessary oath.

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See Ran in Nedarim on the Gemara you linked

הכי הוה ליה למימר דנהי דאמעיטא מצוה מקרבן לא אמעיטא מבל יחל כדחזינן התם בפרק שבועות שתים בתרא (שבועות כה.) דמילתא דליתא בלאו והן אי נמי בלהבא דאמעיטא מקרבן מלהרע או להיטיב ולא אמעיטא ממלקות ולהא לא צריך קרא א״כ מאי קאמר שנאמר נשבעתי ואקיימה ומתרצינן אלא הא קמ״ל דשרי ליה לזרוזי נפשיה כלומר ודאי לא איצטריך האי קרא לומר שהיא חלה לגבי בל יחל אלא ה״ק מנין שדבר הגון הוא להשבע לקיים את המצוה ואפי' כשרים שנמנעין משבועה נשבעין הן בכך שנאמר נשבעתי ואקיימה כלומר שהרי דוד היה עושה כן: ‏

"Mushba Veomed..." means that there is no scriptural prohibition called Shevuat Sheker, leading to Korban Ole Veyored for this lie, because it's a Shevua on a duty, because it's a mitsva.

But regarding an other scriptural prohibition, the Lav of Lo Yachel, it still applies on a Mitsva.

To make an oath to make a Mitsva to motivate yourself to make it is different. The Gemara in Nedarim says that, not only the oath applies , and there is no need to address this in the Gemara, but it's a good practice.

After a survey on the topic in Shevuot 25b and 27a I can summarize that the Mishna in Shevuot addresses sacrifice (Korban) only. An oath to make a positive Mitsva as to eat Matsa on Lel Pesach is an oath, if finally he doesn't eat it, he transgresses a Shevua. Regarding scriptural prohibition for an oath, there are several lavim, see 20b (there is discussion about Shevuat Sheker and Lo Yachel. Regarding the example of the Matsa, the Rambam rules that it's Shevuat Sheker). But regarding the scriptural prohibition called Shevuat Bituy, that involves a Korban, there is a controversy, Machloket, between Rabbi Yehuda Ben Betera and Rabanan in Mishna. Rabanan, and we hold that this is the halacha, say there is no Shevuat Bituy and so no Korban. If someone makes a oath to follow a negative commandment, as the prohibition to eat chamets during Pesach, there is a controversy between Rashi and Riva in Tosfot. The last gives two proofs that Rabbi Yehuda Ben Betera aggrees with Rabanan that there is no Shevuat Bituy and Korban. If someone makes a oath to infringe a mitsva, positive or negative, there is only a Shevuat Shav (Mishna daf 29a, and see Ritba 24a, regarding a Machloket rishonim ) . See Shulchan Aruch YD 236.

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