The Mishnah in Nedarim 79a-b reads:
וְאֵלּוּ נְדָרִים שֶׁהוּא מֵפֵר: דְּבָרִים שֶׁיֵּשׁ בָּהֶן עִנּוּי נֶפֶשׁ: ״אִם אֶרְחַץ״ וְ״אִם לֹא אֶרְחַץ״, ״אִם אֶתְקַשֵּׁט״ וְ״אִם לֹא אֶתְקַשֵּׁט״.
MISHNA: And these are the vows that he, the husband or father, can nullify: The first category consists of matters that involve affliction for the woman who took the vow. For example, if a woman vowed: If I bathe, or: If I do not bathe; if she vowed: If I adorn myself [etkashet], or: If I do not adorn myself.
אָמַר רַבִּי יוֹסֵי: אֵין אֵלּוּ נִדְרֵי עִנּוּי נֶפֶשׁ. וְאֵלּוּ הֵן נִדְרֵי עִנּוּי נֶפֶשׁ: אָמְרָה ״קוּנָּם פֵּירוֹת הָעוֹלָם עָלַי״ — הֲרֵי זֶה יָכוֹל לְהָפֵר. ״פֵּירוֹת מְדִינָה זוֹ עָלַי״ — יָבִיא לָהּ מִמְּדִינָה אַחֶרֶת. ״פֵּירוֹת חֶנְווֹנִי זֶה עָלַי״ — אֵינוֹ יָכוֹל לְהָפֵר. וְאִם לֹא הָיְתָה פַּרְנָסָתוֹ אֶלָּא מִמֶּנּוּ — הֲרֵי זֶה יָפֵר, דִּבְרֵי רַבִּי יוֹסֵי.
Rabbi Yosei said: These are not vows of affliction. Rather, these are vows of affliction: For example, if she said: The produce of the entire world is konam for me as if it were an offering, he can nullify the vow, as it certainly involves affliction. If, however, she said: The produce of this country is konam for me, he cannot nullify the vow, as it does not involve affliction, since he may still bring her produce from another country. Similarly, if she said: The produce of this storekeeper is konam for me, he cannot nullify her vow, as he may still bring her produce from another storekeeper. But if he can obtain his sustenance only from him, that particular storekeeper, he can nullify the vow. This is the statement of Rabbi Yosei.
I am slightly perturbed by the translation of “If I bathe” and “If I do not bathe”, which admittedly follows the Talmudic Narrator’s explanation, and possibly the second generation Amora Rav Yehuda.
Specifically, looking to Biblical language at least, both “im” and “im lo” have identical connotations of forswearing something. That is, in Tehillim 95:11:
אֲשֶׁר־נִשְׁבַּ֥עְתִּי בְאַפִּ֑י אִם־יְ֝בֹא֗וּן אֶל־מְנוּחָתִֽי׃ {פ}
Concerning them I swore in anger,
“They shall never come to My resting-place!”
And in Mishpatim, Shemot 22:7, we have:
אִם־לֹ֤א יִמָּצֵא֙ הַגַּנָּ֔ב וְנִקְרַ֥ב בַּֽעַל־הַבַּ֖יִת אֶל־הָֽאֱלֹהִ֑ים אִם־לֹ֥א שָׁלַ֛ח יָד֖וֹ בִּמְלֶ֥אכֶת רֵעֵֽהוּ׃
if the thief is not caught, the owner of the house shall depose before Godand deny laying hands on the other’s property.
which, as Shadal notes, Rashi, Mendelssohn, and Rosenmuller all take asישבע שלא שלח ידו. So, these can be understood as two divergent languages, both which are a vow or oath not the bathe.
Yet at least the former, im, is taken throughout the ensuing sugya — which is all the presumably Savoraic Talmudic Narrator — to refer to a conditional, and they grapple with how that works.
Amoraim only come in when grappling with the interpretation of “im”, as to how it would work with the corresponding “im lo”, on 80b:
שַׁנֵּית ״אִם אֶרְחַץ״. ״אִם לֹא אֶרְחַץ״ הֵיכִי דָּמֵי? אִלֵּימָא דְּאָמְרָה: ״תִּיתְּסַר הֲנָאַת רְחִיצָה לְעוֹלָם עָלַי אִם לֹא אֶרְחַץ הַיּוֹם״ — לְמָה לָהּ הֲפָרָה? תִּתְסְחֵי!
אָמַר רַב יְהוּדָה: דְּאָמְרָה ״הֲנָאַת רְחִיצָה עָלַי לְעוֹלָם אִם לֹא אֶרְחַץ בְּמֵי מִשְׁרָה״.
The Gemara asks: You have adequately answered the expression: If I bathe, but as for the vow: If I do not bathe, what are the circumstances? If we say that she said: The benefit of bathing shall be forbidden to me forever if I do not bathe today, why does she need nullification at all? Let her bathe today and nothing will be forbidden. Rav Yehuda said: The mishna is referring to a case where she said: The benefit of bathing is forbidden to me forever if I do not bathe in foul water in which flax was soaked. The husband can nullify this vow, as it will make her repulsive, which is a form of disfigurement.
דִּכְווֹתֵיהּ דְּקָתָנֵי ״אִם לֹא אֶתְקַשֵּׁט״ — אִם לֹא אֶתְקַשֵּׁט בְּנֵפְטְ. לִכְלוּךְ הוּא!
The Gemara raises an objection: In that case, you must similarly explain that which the tanna teaches: If I do not adorn myself, to mean: The benefit of adorning myself is forbidden to me forever if I do not do something repulsive, e.g., if I do not adorn myself with naphtha [neft]. But this cannot be, as such a substance is filthy and the term adornment cannot be applied to it at all.
אָמַר [רַב יְהוּדָה], דְּאָמְרָה: ״הֲנָאַת רְחִיצָה לְעוֹלָם עָלַי אִם אֶרְחַץ הַיּוֹם, וּשְׁבוּעָה שֶׁלֹּא אֶרְחַץ״, ״הֲנָאַת קִישּׁוּט עָלַי לְעוֹלָם אִם אֶתְקַשֵּׁט הַיּוֹם, וּשְׁבוּעָה שֶׁלֹּא אֶתְקַשֵּׁט״.
Rather, Rav Yehuda said that the mishna is referring to a case where she said: The benefit of bathing is forbidden to me forever if I bathe today, and I take an oath that I will not bathe today. Through a combination of her vow and her oath she has rendered it prohibited for her to bathe forever. The situation is similar if she said: The benefit of adornment is forbidden to me forever if I adorn myself today, and I take an oath that I will not adorn myself today.
Rav Yehuda’s latter explanation, in its second part, corresponds to my interpretation. Namely, that “im lo” is not a conditional. The first part still takes “im” as a conditional. But early Amoraic statements were often short, and so perhaps this was lengthened by the Talmudic Narrator. Ravina and Rav Ashi’s conversation that ensues also understands at least the “im lo” as being oath language, so perhaps we could apply that to the “im” language as well.
How about Rav Yehuda’s former explanation? That seems to definitely have a conditional as part of it? I have nothing, other than an unsubstantiated conjecture that she is not declaring a conditional, but saying “I won’t bathe chutz, except for in flax-soaked water”, which may then feed well into the seifa of the Mishnah with Rabbi Yossi’s discussion and whether there is a valid alternative path.
I also wonder how this corresponds to niddah rules, for how shall she immerse in a valid mikveh? This interpretation wouldn’t run afoul of Rava’s discussion about innuy, because of Yom Kippur’s five inuyim, tashmish hamitah is also not one of the karet-bearing ones, just eating and drinking, and prohibited labor. If so, it might work will as bein ish le’ishto as well as inuy nefesh.
On to the manuscript variants! Regarding the latter explanation of Rav Yehuda, Rishonim and various manuscripts attribute this to Rava. Thus, Marburg and Vatican 110 both have אלא אמר רבא.
Marburg doesn’t state Rav Yehuda for the former the above image, but that is likely because so much is scrubbed out at the top line, so it cannot be made out.
Munich 95, meanwhile, only has Rav Yehuda for the former statement and א”ל, perhaps amar lecha, that “Rav Yehuda would say to you”, for the latter statement. The apostrophe is also between the aleph and lamed, not after, which would make it perhaps a shortening of the אלא that we see in other manuscripts (but not printings). Even though the latter statement is a significant revision of the former, rather than a defense. That variant thus seems incorrect. If it were elah, it is a revision, but perhaps then attributed to the Stamma, or implicitly to Rav Yehuda.
Indeed, Shadal doesn't like im lo as shelo either. Though we can add Rashbam to shelo side, and Rashi on Shas.
Still, given that enough commentators, including peshat commentators, could take it as such, I'd credit it as a strong explanation of the Mishnah.
BTW, in the Yerushalmi parallel, we do have more named Amoraim giving the conditional interpretation. https://www.sefaria.org/Jerusalem_Talmud_Nedarim.11.1.8?vhe=The_Jerusalem_Talmud,_edition_by_Heinrich_W._Guggenheimer._Berlin,_De_Gruyter,_1999-2015&lang=bi&with=all&lang2=en