Siman 266

The lost property of oppressors is permitted, as it is said, "the lost property of your brother." One who returns it to him strengthens the hands of transgressors. However, if he returned it in order to sanctify the Name of Heaven, so that they should glorify Israel as being people of integrity, he is praiseworthy and has sanctified the Name. Therefore, in a place where there is a desecration of the Name if he does not return it, he is obligated by law to return it. In all places, we bring in their vessels because of thieves, just as one is obligated to do for an Israelite, for the sake of the ways of peace.

One is obligated to return the lost object of a Jew even if the owner of the lost object was a wicked person and eats nevelot out of desire. However, one who eats nevela to provoke is an apikorus from Israel; and the apikorsim, and idolaters, and a Jew who desecrates Shabbat in public, it is forbidden to return a lost object to them, for such people rebel against the God of Heaven and the King of the Earth, as it is written: "Fear the Lord, my son, and the King," for anyone who fears the Lord also has upon him the fear of the King, and likewise the opposite.

One who is forbidden by a vow from deriving benefit from his fellow may return his lost object to him, whether the property of the one returning it is forbidden to the owner of the lost object, or whether the property of the owner of the lost object is forbidden to the one returning it, or even when the property of both of them is forbidden to one another. The reason was explained in Yoreh Deah siman 221. In a place where payment is taken for the returning of a lost object, the payment shall go to the hekdesh, as was written there.

A dangerous cat or a dangerous dog that harms small children is not subject to the laws of returning a lost object; rather, anyone who finds them is commanded to kill them and acquires their hide, for they are ownerless, and the owners who maintain them commit a transgression.

If his father said to him, "Do not return the lost object," he should not listen to him, for regarding the transgression of a prohibition, even a rabbinic one, there is no honor of the father in this, since the father is also obligated in the commandments of the Holy One, Blessed be He. And thus our Sages, of blessed memory, expounded: "Every man shall revere his mother and his father, and you shall keep My Sabbaths; I am Hashem" — you are all obligated in My honor. And if his father said to him to desecrate the Shabbat or any other transgression, it is forbidden to listen to him. And not only this, but even if his father said to him, "Do not return the lost object and occupy yourself with my needs," which is the essence of honor of the father, nevertheless, returning the lost object takes precedence because honor of the father is a positive commandment, while returning a lost object is a positive commandment and a negative commandment. And if he is occupied with honor of the father and a lost object came his way, he is exempt from returning it, for one who is occupied with a mitzvah is exempt from another mitzvah [Taz].