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MB3 193a: Haforas Nedarim on Shabbos (341:1-3)
If you made a neder that affects Shabbos (e.g. not eating), you can do הפרת נדרים on Shabbos. הפרת נדרים has to be done with a שינוי but התרת נדרים via a חכם doesn't require a שינוי.
What if a person makes a neder to do a specific מלאכה by a specific date which happens be Shabbos and he still hasn't done it yet (and can't because it's Shabbos)?