Sforno on Genesis 14:23/ספורנו על בראשית י״ד:כ״ג

1אם מחוט ועד שרוך נעל, every time the word אם occurs in Scripture and is not followed by a verb making it a conditional statement, 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.