Nedarim 27a: If one was under duress in his failure to fulfill a vow, it is permitted. For example, if Reuven made Shimon swear that he would eat at his house, and then Shimon got sick, or his son got sick, or a river blocked his path – this is called duress.

Yoreh Deah 232:12, Rema: Some say that the exemption of duress applies only when he had to swear to avoid something bad happening to him, not if he merely swore to gain something.

נדרים כ ע”ב: ארבעה נדרים התירו חכמים: נדרי זרוזין, ונדרי הבאי, ונדרי שגגות, ונדרי אונסין. ושם כז ע”א: נדרי אונסין: הדירו חבירו שיאכל אצלו, וחלה הוא או שחלה בנו, או שעכבו נהר ־ הרי אלו נדרי אונסין.

יו”ד רל”ב,יב: נדרי אונסין כיצד, הדירו חבירו שיאכל אצלו וחלה או שחלה בנו או שעכבו נהר הרי אלו נדרי אונסין… רמ”א: ויש אומרים דלא מקרי אונס ולא מקרי אונס בשבועה אלא א״כ היה מגיע לו איזה רעה אם לא היה נשבע אבל אם לא היה מגיע לו טובה ונשבע כדי שיהיה לו טובה מזה אין מקרי אנוס בשבועה (מהרי״ק שורש קס״ז ופסקי מהרא״י סי׳ ע״ג).

Born in 1930 in Kaschau, Hungary, Rabbi Avraham Weinfeld was 14 years old when the Germans deported his entire town to Auschwitz. His parents, siblings and extended family were all killed immediately. Soon he was transferred to a labor camp and forced to work all winter in sub-freezing temperatures, wearing nothing but a thin prison shirt. For a time, he had typhus. He later wrote in the hakdama to his sefer Even Yechezkel that he took a vow while in the camp that if he survived, he would dedicate his entire life to learning and teaching Torah. This was in spite of the fact that he came from a family that was not particularly scholarly. His father and grandfather had owned a factory and were well-to-do people. The idea of dedicating his life to Torah was his own.

After the war, he came to America and learned under R’ Moshe Feinstein at MTJ. When he became engaged, he and his kallah went visit to the Satmar Rebbe, who asked him what he would be doing for a living. Reb Avraham responded that he was going to learn. “What are you going to live on?” asked the Rebbe. “I made a neder in the concentration camps that if I would survive, I would dedicate my life to learning Torah,” said Reb Avraham. The Rebbe made a dismissive gesture and said, “That’s nidrei onsin. You don’t have to keep it.” But Reb Avraham  did not give up.

He bought a newspaper, looked at the classifieds and chose an apartment in Harlem (unaware that, by that time, Harlem was no longer a thriving Jewish neighborhood). The first day of Sheva Brachos, he went to daven Mincha and Maariv in a local shul. After Mincha, a few old men approached him and asked him if he could give a shiur on Mishnayos. He agreed and gave the shiur. After Maariv, the shul members asked him if he would become their Rav. For ten dollars a month, he became the Rav of that shul. Eventually, another shul hired him, paying him twelve dollars a month; he alternated between the shuls and that was how he made a living.

In that first shul, the women’s balcony had no mechitzah, but no women really came to shul so there wasn’t much of an issue. As the high holidays approached and women were to come, he told the people in the shul that they needed a mechitzah. “This shul is a hundred years old,” the members responded, “and this is the way we’ve been davening for all these years. We’re not interested in a nineteen year old boy telling us how to daven.” “Well,” he said, “if there’s no mechitzah, I can’t daven here.” So they came up with a solution: they built a mechitzah just around Reb Avraham’s seat.

On Yom Kippur night, to his surprise, he saw everyone in the shul wearing leather shoes. During his drashah before Kol Nidrei, he spoke about the prohibition to wear shoes. They all listened to him and removed their shoes, except for the president of the shul, who refused. In the morning, Reb Avraham came back to shul and found that his mechitzah was gone. The president had taken it down. “The way you are living is not how people live today,” said the president. “No,” said Reb Avraham. “If a person lives according to the Torah, he lives, and if he not, it’s not a life.” He left the shul and walked, on Yom Kippur morning, from Harlem to Williamsburg to daven by the Klausenberger Rebbe.

When he came back home Motzaei Yom Kippur, he found all the people of the shul gathered in the hallway of his apartment. They told him that that morning, the president of the shul had dropped dead in the middle of davening. Terrified, they asked him for mechilah. Reb Avraham told them that it had nothing to do with him. He didn’t stay much longer in that position.

Source: https://ravweinfeld.com/posts/bo/#yartzeit-of-reb-avraham-weinfeld

[Based on the Mechaber and Rema, there are two conditions needed to be considered “nidrei onsin”: the vow was made due to fear of something bad happening, and the fulfillment of the vow became too difficult because of a new situation that arose, e.g. he got sick. In our case, the vow was made because the young Avraham Weinfeld was afraid of dying in the Holocaust, and he wanted Hashem’s protection, so the first condition is satisfied. But what was the new situation that arose? He knew all along that it would be difficult to sit and learn his whole life (although perhaps we could say that he didn’t know how exactly difficult it would be).

Furthermore, it’s not even clear that this is called “a vow taken to avoid something bad.” Perhaps it should be viewed the opposite way: most Jews did not survive the camps. In all probability, he would die too. He was asking Hashem for special protection, and in return he promised to sit and learn. Maybe this is similar to the Rema’s case of swearing in order to gain something.

Indeed, the Ramban on Vayikra 22:18 says:

כי הנדר הוא הבא על דבר שיפלא ממנו, שידור לה׳ בצר לו אם תעשה עמי להפליא להצילני מן הצרה הזאת אביא עולה או שלמים, כענין וידר יעקב נדר לאמר אם יהיה אלהים עמדי (בראשית כח כ) , וידר ישראל נדר לה׳ ויאמר אם נתון תתן (במדבר כא ב) , וידרו נדרים (יונה א טז(.

The word “yafli” is used in connection with a neder because a person usually makes a neder in order to merit help from Hashem in something that is too hard for him (“pele” means hidden, beyond, wondrous). He vows to Hashem in his time of trouble, “If you do wonders for me and save me from this danger, I will bring a korban olah or shlamim.” For example, “Yaakov vowed a neder saying, if G-d will be with me…” “Israel vowed and neder to Hashem and said, if You deliver this people into my hands…” And regarding the sailors on the ship with Yonah, it says, “They made vows.”

Is it conceivable if Yaakov Avinu, the Bnei Yisroel or the sailors had encountered some inconvenience, they would not have had to keep their vows?

On the issue of the mechitzah, it’s interesting that according to Reb Moshe Feinstein (Igros Moshe OC 1:39), the high balcony would have been sufficient, as it prevents mingling; preventing visibility is not required. This teshuva was written in 1946, before our story. Perhaps the case was that “balcony” was almost level with the main shul. Alternatively, although Reb Avraham Weinfeld had learned under Reb Moshe, he was more stringent regarding the requirement of mechitzah.

If Reb Avraham held that the mechitzah needed to block visibility, then why would a mechitzah just around his seat help? The rav may as well have put on a blindfold; the shul would still not be following the halachos of a shul because they didn’t have a kosher mechitzah, and one may not daven in such a shul. If a blindfold doesn’t help, why does a mechitzah around his seat help? The answer is that the mechitzah rendered his seat a different domain, not part of the shul at all. As to the rest of the shul, he was willing to let them follow Reb Moshe’s shitah.]