Shevuah worded in the negative

Question

I've been bothered for a long time: Throughout Tanach, shevuos (oaths) and brisos (covenants) are phrased in the negative (where it doesn't just say, I swear __, like בי נשבעתי ___). Generally language indicating acceptance of some major punishment if the oath is violated, such as כה יעשה ה' לי וכה יוסיף, followed by אם and some condition that - if violated - will bring the punishment.
One example is in this week's parsha (Bereishis 31:48-50):

וַיֹּאמֶר לָבָן הַגַּל הַזֶּה עֵד בֵּינִי וּבֵינְךָ הַיּוֹם כו' וְהַמִּצְפָּה אֲשֶׁר אָמַר יִצֶף ה' בֵּינִי וּבֵינֶךָ כִּי נִסָּתֵר אִישׁ מֵרֵעֵהוּ: אִם־תְּעַנֶּה אֶת־בְּנֹתַי וְאִם־תִּקַּח נָשִׁים עַל־בְּנֹתַי אֵין אִישׁ עִמָּנוּ רְאֵה אֱלֹהִים עֵד בֵּינִי וּבֵינֶךָ "Let this pile be a witness between us today... and this lookout, that Hashem should look between us when we are hidden from one another... If you make my daughters suffer, if you take other wives besides my daughters... Hashem will see..."

In other words, Yaakov is agreeing not to do those things. And this is the norm in all the many dozens of such shevuos in Tanach.
So why is this rule broken, just a couple of lines later (ibid:52):

עֵד הַגַּל הַזֶּה וְעֵדָה הַמַּצֵּבָה אִם־אָנִי לֹא־אֶעֱבֹר אֵלֶיךָ אֶת־הַגַּל הַזֶּה וְאִם־אַתָּה לֹא־תַעֲבֹר אֵלַי אֶת־הַגַּל הַזֶּה וְאֶת־הַמַּצֵּבָה הַזֹּאת לְרָעָה
"Let this pile be a witness, this lookout be a witness, if I do not cross this pile to you, if you do not cross this pile and this lookout to me, to do ill."

They ought to be promising that neither should cross over to make war on one another. But the wording sounds like the opposite of the usual rule. Why? And are there other counter-examples?

Answer 1

Direction for an answer? B"h, found a Seforno who backs up my claim, and says something about an exception (Bereishis 14:23):

אם מחוט ועד שרוך נעל כל אם שבמקרא כשלא יבא אחריו פועל תנאי יהיה במקום מלת שלא אמר א''כ הרימותי ידי שלא מחוט ועד שרוך נעל אוכל ליתן לך שאין בידי כלום ושלא אקח מכל אשר לך. וכן אם יראו את הארץ שלא יראו. אם אתם תבאו שלא תבאו. חי ה' אם יומת שלא יומת. חי ה' אשר עמדתי לפניו אם אקח שלא אקח וזולתם רבים
אם מחוט ועד שרוך נעל, every time the word אם occurs in Scripture and is not followed by a verb making it a conditional statement [ital. mine], it is used instead of the word שלא, “so that not.” Avram’s statement here means: “I have sworn an oath not to give you even a shoelace or a length of thread as I do not own any of these things. Conversely, I will also not take (accept) anything that is (was) yours.” G’d said something similar to the Jewish people after the debacle of the spies in Numbers 14,23) when He said אם יראו את הארץ, which is a way of saying that they will most certainly not get to see the land (of Israel). In verse 30 in the same chapter the words אם אתם תבאו also mean “you will certainly not come.” Similarly, Samuel I 15,6 חי ה' אם יומת, means “an oath to G-d that he will not be executed, etc.” Also, in Kings II 5,16 'חי ה' אשר עמדתי לפניו אם אקח וגו, means: “As the Lord lives whom I serve, I will not accept anything.” There are many similar examples. [See also the Seforno on Bamidbar 14:23.]

His words, "and is not followed by a verb making it a conditional statement", might apply to our case? I am not sure how you tell, though; all these cases have a verb and seemingly could be considered conditional. He didn't give an example of that type.