Even doing something in his own domain something which causes damages to his neighbor is prohibited. He should not place in his courtyard, adjacent to his friend’s wall, anything which has [and gives off] heat, or gives off gases, or would [otherwise] damage the wall, such as manure and the like, unless if he keeps it a distance of 3 tefachim [roughly 9"; enough to keep the heat or fumes from damaging the wall]. Similarly, he has to keep distant so as not to spill water adjacent to his friend’s wall. Therefore, a gutter which drains [water] from the roof, he must distant it[s outlet] 3 tefachim from his friends wall. All the more so not to spill a chamberpot of urine adjacent to his friend’s wall. To urinate adjacent to his friend’s wall, if it is a wall of stone of of wood without tarring [or plaster], it is enough to keep a distance of 1 tefach [3" or so]. If it is of stone fitted into stone, he doesn’t have to keep any distance, and can even urinate against the wall. If it is a brick wall, or of tarred [or plastered] wood, he has to keep a distance of 3 tefachim. See more in the next siman [184, which deals with physical damages; ie injury].
It is prohibited to enter his friend’s plow-field because you will trample what he plowed and ruin it
It is prohibited to stand over his friend’s field to look it over while it if standing at full height [ready for harvest], so that ayin hara* does not damage it. All the more so one should not look into [the affairs of] a friend in some matter that one can fear that ayin hara will harm him.
Even in his dealings and actions about which there is no fear of damage through ayin hara, if he does it in his house and his property, it is prohibited to watch without his knowledge, because maybe he doesn’t want others to know of his actions and dealings. [I.e. you are obligated to grant others privacy.]
It is proper conduct that when someone sees his friend busy at his work to bless him and say, “May you succeed in your doings!”
* Ayin hara, litetally “evil eye”, is a term used in the mishnah for jealousy (e.g. Avos 5:19). Since Hashem generally punishes through the means by which the person sinned, a logical punishment for arousing jealousy in others through unnecessary conspicuous consumption is to lose the item in question.
Therefore, even if someone is only somewhat demonstrative about his wealth, and thus you would not grow jealous without taking some effort on your own to look over his success, he is better off if you do not do things to grow jealous of it. This way he will not be punished for his share in causing you that pain.
If it prohibited to surrender on a Jew to the control of non-Jews, whether bodily or with respect to his money, whether through action or speech, to inform on him or to reveal his hiding place. Whomever surrenders [a fellow Jew] has no place in the World to Come. Even someone who is evil and a master of sin, it is prohibited to turn him over, neither his body or his money. Even if he is a troublemaker to him and causes him problems constantly with his words. However, if his friend turns him in, and there is no way to save himself except through turning in [the informer] it is permitted.
None of the above applies to eliminating someone who is dangerous to the community who can not be removed in some other way. In other words, while informing on a fellow Jew to non-Jewish authorities is prohibited, allowing a child molester (for example) to continue posing a danger to our children is a greater prohibition. If someone finds themselves in this situation, one must consult with communal leaders to see if the authorities are a necessary means of getting him off the streets, and if so, speak with rabbis about the permissible way to do so.
Similarly, if the king’s army enters the city and the people of the city are obligated to provide them hospitality, one is prohibited to pay off the general of the troups to get him out of it. Because through this he will cause damage to someone else. Similarly in all other matters of taxes, it is prohibited to make efforts by the general to release him, if through this [the tax] will be heavier on others. And one who does this is called an “informer”.
Even someone who incurred damages on himself he can not remove it from himself if through this he will cause [damages] to come upon someone else. Because he can’t save himself even through indirect causing his friend monetary damages. However, before he incurs the damages, he may push it off that it won’t come upon him, even if through this it will be incurred by his friend. For example, if a stream of water approached [threatening] to flood his field, while it hadn’t yet entered his field, it it permitted to place a damn before it even though through this it will flood his friend’s field. However, once it entered his field, it is prohibited to remove it in a way that it would enter someone else’s. Since the damage already occured, it is inappropriate to divert it from him and place it on his friend.
All else being equal, one may save their own first. However, if I already incurred damages, then all else isn’t equal. I’m merely using my misery as an excuse to share it with someone else.
סִימָן קפג – הִלְכוֹת נִזְקֵי מָמוֹן
183: Laws of Fiscal Damages
It is prohibited to cause financial damage to his friend, even with the intent to repay, just as it is prohibited to rob or to steal with intent to repay. Even causing damage to his friend, whether by act or by speech, is prohibited. For example: Re’uvein who sells merchandise to a non-Jew. Comes Shim’on and says to him [the non-Jew] that it is not worth so much — even though it is true, it is prohibited, for this overpayment is permitted. [Unlike closing a deal in which a Jew willingly overpays, which is not.] Whomever causes damages to his friend, even in a matter which is not punishable by human law, he is obligated by the law of [the One in] heave, until he appeases his friend.
In the the Qitzur’s example, the Jew’s actions qualify Re’uvein as what the Rambam would call “naval bireshus haTorah — disgusting with the permission of the Torah.” The deal is morally wrong, but not so much so as to be the subject of a Torah prohibition which would apply to all people at all times. Therefore, undermining the deal would be fiscal damages, even though the deal is not, deep down, even in Re’uvein’s spiritual best interest. The correct response would be to get the Jew not to take advantage of the non-Jew’s ignorance or naivite. Not to work against him.