Does the בְ in לֹֽא־תַעֲנֶ֥ה בְרֵעֲךָ֖ עֵ֥ד שָֽׁקֶר only mean against?

Question

Does the בְ in לֹֽא־תַעֲנֶ֥ה בְרֵעֲךָ֖ עֵ֥ד שָֽׁקֶר only mean against or can it mean about with the idea of claiming the innocence of a guilty person as well as accusing an innocent person? Or, maybe among is better.

Answer 1

I'm not sure how you can read it in a positive way? Perhaps you can add to your question to make it clearer what you mean.

However as far as understanding the use of the letter "ב" it is worth noting the Ibn Ezra who writes the following:

לא תענה. שנים רבות חפשתי בלבי טעם זאת המלה לאמר עד שקר ולא אמר עדות שקר. ולפי דעתי עתה כי הוא מדבר עם עד שקר. כאילו כתב לא תענה אם אתה עד שקר ומשמע זאת המלה כטעם לא תכחשו ולא תשקרו. ומצאנו מלת ענות בבי"ת. כמו שקר ענה באחיו. ובלא בי"ת כמו ישמע אל ויענם. והטעם יענה בם. וכמוהו וכאשר לא נגענוך. לא נגענו בך. ויש עד שקר שהוא בן מות. והוא שכתוב עליו ועשיתם לו כאשר זמם לעשות לאחיו:

THOU SHALT NOT BEAR FALSE WITNESS. For many years I asked myself why Scripture employs the phrase ed sheker (false witness) and not edut sheker (false testimony). I now believe that Scripture is addressing the false witness, and our verse is to be interpreted as if it read, “Thou shalt not give testimony if thou art a false witness.” The implication of this clause is the same as that of neither shall ye deal falsely, nor lie one to another (Lev. 19:11). We find that a bet follows the word for testimony in and hath testified (anah) falsely against his brother (be-achiv) (Deut. 19:18). We also find this verb employed without a bet following it in yishma El ve-ya’anem (God shall hear, and testify against them) (Ps. 55:20). Now ve-ya’anem is a compound of ve-ya’aneh (and shall testify) and bam (against them).273 The phrase lo nega’anukha (not touched thee) in as we have not touched thee (Gen. 26:29) is similar, as the meaning of lo nega’anukha is lo naganu bakh (we have not touched thee). There is a type of false witness who incurs the death penalty, namely, the one concerning whom it is written, then shall ye do unto him, as he had purposed to do unto his brother (Deut. 19:19). (sefaria translation)

The relative openness of the command which I think you are finding difficult to understand as far as which we to read the injunction is addressed by Rabbeinu Bachya who notes that the ambiguous wording is designed to maintain a universal application of the negative impact of being a false witness. As he writes:

לא תענה ברעך. לא אמר באחיך כמשפט הכתובים (דברים כ״ג:כ׳) לא תשיך לאחיך, (שם כב) לכל אבדת אחיך, וכיוצא בזה, כי שם אנו דורשים אחיך ולא עכו"ם וכאן הזהיר אפילו על העכו"ם כי כן מצינו (שמות י״א:ב׳) וישאלו איש מאת רעהו והוא נאמר על המצריים, ועוד שאלו אמר באחיך היה נראה שהעדות כשרה בקרובים כי לא יזהירנו אלא מעדות שקר אבל עדות של אמת כשר ואין הדבר כן כי כל עדות פסולה בקרובים הן שיהיה שקר הן שיהיה אמת הן יהיה לתועלתו או להפסדו. והיה הכתוב ראוי לומר לא תענה ברעך עדות שקר, ואלו אמר כן לא היתה האזהרה כי אם למעיד עצמו שלא יעיד הוא עצמו עדות שקר, אבל כשאמר עד שקר יתבאר מזה שהוא אזהרה שלא יהיה נסכם להזמין עד שקר, וזהו לשון לא תענה לא תסייע ולא תזמין ברעך עד שקר שיעיד עליו.

לא תענה ברעך עד שקר, “do not bear false witness against your fellow.” Please note that the Torah does not write “against your brother,” as it does when forbidding charging interest, (Deut. 23,20) or when it commands you to return found property (Deut. 22,3), or in similar legislation. In all those instances our sages explain the word אחיך as excluding non-Jews. In other words, while it is forbidden to charge (or pay) interest to a Jew, it is not forbidden to charge interest to a Gentile. While one must go out of one’s way to restore lost property owned by a Jew, there is no legal obligation to spend time, energy, and even money to restore lost property to a Gentile. Here the reason the Torah fails to limit the legislation to אחיך, “your brother.” It is clear that one must not bear false testimony against a Gentile, i.e. against Egyptians. Moreover, we find in Exodus 11,20 וישאלו איש מאת רעהו, “they asked each person from his fellow;” it is clear that the word רעהו refers to the Egyptians as the verse would not make any sense otherwise. Not bearing false testimony therefore is a commandment of universal application. Furthermore, if the Torah had written the word אחיך in our verse we might have concluded that it is in order to testify against (or on behalf of) other relatives, whereas generally speaking a whole list of relatives are unfit to testify together (as a team), on behalf of each other or against each other. Not only is false testimony illegal in such instances but even true testimony is unacceptable. Were this not so the Torah should have written לא תענה ברעך עדות שקר, “do not lie in your testimony against your fellow.” Such wording would have been very misleading as it would have meant that only the actual false testimony is prohibited but that someone who is in collusion with people arranging false testimony would be exonerated by the legal process. The Torah therefore uses ambiguous wording to cover as much ground as possible, i.e. in order to include as many people as possible in the parameters of guilt governing the subject of false testimony. The very word תענה means to “assist,” or “invite.” (Sefaria translation)