(7) Distance yourself from falsehood; and do not kill the innocent and righteous; for I will not exonerate the wicked.
רבן שמעון בן גמליאל אומר, על שלשה דברים העולם עומד, על הדין ועל האמת ועל השלום, שנאמר (זכריה ח) אמת ומשפט שלום שפטו בשעריכם .
Rabban Shimon ben Gamliel used to say: On three things does the world stand: On justice, and on truth, and on peace, As it is said: “Judge with truth, justice, and peace in your gates.”
(יב) וַתִּצְחַ֥ק שָׂרָ֖ה בְּקִרְבָּ֣הּ לֵאמֹ֑ר אַחֲרֵ֤י בְלֹתִי֙ הָֽיְתָה־לִּ֣י עֶדְנָ֔ה וַֽאדֹנִ֖י זָקֵֽן׃ (יג) וַיֹּ֥אמֶר יקוק אֶל־אַבְרָהָ֑ם לָ֣מָּה זֶּה֩ צָחֲקָ֨ה שָׂרָ֜ה לֵאמֹ֗ר הַאַ֥ף אֻמְנָ֛ם אֵלֵ֖ד וַאֲנִ֥י זָקַֽנְתִּי׃
(12) And Sarah laughed within herself, saying: ‘After I have grown old shall I have soft skin, and my husband is old?’ (13) And G-d said to Abraham: ‘Why does Sarah laugh, saying: Shall I indeed give birth, I am old?
נודרין להרגין, ולחרמין, ולמוכסין, שהיא תרומה, אף על פי שאינה תרומה; שהן שלבית המלך, אף על פי שאינן שלבית המלך.
One may vow to murderers and thieves and [self-appointed] tax collectors that [something] is terumah (the tithe to the cohen), even if it isn't terumah; [or] that they are property of the king, even if they aren't property of the king.
Rabbi Eliyahu Dessler, Michtav M’Eliyahu, Vol. I, p. 94
What is truth (אמת) and what is falsehood (שקר)? When we went to school we were taught that truth is to tell facts as they occurred and falsehood is to deviate from this. This is true in simple cases, but in life many occasions arise when this simple definition no longer applies.
Sometimes it may be wrong to “tell the truth” about another person, for example if it would reveal something negative about him, unless there was an overriding purpose and necessity. And sometimes it may be necessary to change details, when the plain truth would not bring benefit, but injury. In such cases what appears to be true is false (שקר), since it produces evil effects; and what appears to be false may help to achieve the truth.
We had better define truth (אמת) as that which is conducive to good and which conforms with the Will of the Creator, and falsehood (שקר) as that which furthers the scheme of the Yetzer HaRah, the power of evil in the world.
דאמר רב יהודה אמר שמואל בהני תלת מילי עבידי רבנן דמשנו במלייהו במסכת ובפוריא ובאושפיזא
Rav Yehuda said in the name of Shmuel, "In only these three matters is it the practice of the rabbis to deviate in their speech [from the truth]: regarding a section of Torah, intimacy, and hospitality.
באושפיזא - שאלוהו על אושפיזו אם קבלו בסבר פנים יפות ואמר לאו מדה טובה היא כדי שלא יקפצו בו בני אדם שאינן מהוגנין לבא תמיד עליו ויכלו את ממונו:
Regarding hospitality - If people ask him if his host received him well. Answer them "no." This is a good character trait in order to prevent undesirable guests from inundating him [the host] and exhausting his resources.
דבי רבי ישמעאל תנא: גדול השלום שאף הקדוש ברוך הוא שינה בו: דמעיקרא כתיב (בראשית יח, יב) ואדוני זקן ולבסוף כתיב ואני זקנתי
The school of Rabbi Yishmael taught in a beraisa "Great is peace, for even The Holy One, Blessed is He alters [the truth] for its sake. Originally it is written "And my husband is old!" And in the end it is written "And I am old!" (Bereishit 18:12-13)
ואדוני זקן - שרה אמרה לשון בזוי וכשגלה הקב"ה הדבר לאברהם כתיב למה זה צחקה שרה וגו' ואני זקנתי ושינה הדבר מפני השלום:
"And my husband is old!" - Sarah used degrading speech. And when the Holy One, Blessed is He, revealed the matter to Abraham, it is written "Why did Sarah laugh?... and I am old!" He altered the truth for the sake of peace.