GEN940 The degree of importance that the Torah attaches to Derech Eretz [proper conduct] is demonstrated by the fact that, in some circumstances, it endorses the use of bending the truth in order to maintain peaceful relations between people.  This is derived from the dialogue between the Almighty and Avraham Avinu.   When Sarah heard the angel proclaim that she would be blessed with a child, she laughed [this verse].   When Hashem reported Sarah’s reaction to Avraham, however, He modified her words, and said, “Why did Sarah laugh, saying, ‘Shall I, who am old, indeed bear a child?”  (ibid. 13).   Rashi explains that Hashem omitted Sarah’s statement, “…my lord being old also,” in order to prevent dispute between husband and wife.  The Talmud Ketubot 17a derives from this verse that it is permitted to congratulate a person for making a wise purchase even if the purchase was in truth unwise; likewise it is permitted to say that an ugly bride is beautiful.   In general, any situation that, if left unchecked, would result in the severance of friendly relations warrants bending the truth.   Yosef’s brothers said to him after their father’s death, “… Your father commanded before his death saying, “…Please pardon your brothers’ sin…’” Genesis 50:17.   Rashi explains that Yaakov never made such a command; he did not suspect that Yosef would harm his brothers.   In fact, the brothers fabricated this account in order to ensure their own safety.  This teaches that it is even permitted for one of the contending parties to bend the truth if it will help restore harmony.   The Talmud Yevamot 65b states that it is not only permitted to bend the truth for the sake of Derech Eretz, but it is even a mitzvah to do so.   WAGS 112-3