KEEPING TO TRUTH

One is allowed to change the truth for the purpose of peace. However, this only applies when relating something about the past, but not when promising something that is to be done in the future.

However the Baal Shem Tov cautions that one who often speaks falsehood should abstain from changing the truth for the purpose of peace, since this may strengthen his bad habit.

(שו"ע אדה"ז סי' קנ"ו, כתר שם טוב אות מ)

A wealthy man decided one night, while saying Kerias Shema, that he would give twenty-five rubles to tzedaka every time he said something that was not true. When the local melamed heard this, he exclaimed, "Then lie and provide for the poor!"

Later, when that melamed visited Lubavitch, the Rebbe Maharash rebuked him for his advice, for one is not allowed to lie, even for a good reason.

(סיפורי חסידים זוין תורה ע' 208)