Must you inform someone if you know you could save them from losing money? For example, if you work in an office building, and you realize a window which should be shut is open and resulting in significantly higher heating bills for the office building, must you inform the owner of this?
According to R Tzvi Spitz in his book, Cases in Monetary Halacha, yes you do. He brings the following points
- Just as there is a mitzvah to return lost objects to their owner, so it is obligatory to save a fellow Jew from incurring a loss of any kind if one is able to do so
- One acting in this manner has fulfilled a Torah commandment and, conversely, if he has an opportunity to do so and and doesn't he has transgressed a positive mitzvah of the Torah (according to the Rambam's Sefer Hamitzvot [negative 297] it is a transgression of "You shall not stand by idly over your fellow man's blood")
- Therefore whenever one sees an air conditioner [...] left on when not needed he is obligated to remedy the situation or notify the owner
- The only limitation is if it will result in monetary hardship or physical suffering as one is not obligated to sacrifice his own money to save the property of someone else. But mere unpleasantness is not enough to exempt oneself from following a mitzva.
Maybe you should close the window (that is what I will do).
(But it might be open for a reason, so maybe ask him (but obviously do not teach him how to run his building (it is disrespectful, not nice)))
Rabbi Yosi says: The money of your friend should be as dear to you as your own.