It seems clear from the Mishnah and from Rav Nachman that "maot mfuzarim" is the property of the "finder."
Also, Choshen Mishpat, Chalek 3, 262:6 states this quite clearly. Logically then, there would not even be an obligation of "Lifnim Meshurat HaDin" to announce that a certain sum of money has been found. Yet, if we look in the same "sefer" section 229:7 the RAMA says that "nonetheless, it is good and it is proper to return the lost object" (This RAMA appears in parentheses so it is unclear if this is actual RAMA or not)-seemingly indicating the obligation to at least announce the find. With all of this said, what is the actual "halacha l'maaseh" if one finds money without a "siman" in a Jewish neighborhood or in a yeshiva? What ,if anything, is a person obligated to do?
Rabbi David Etengoff, Brooklyn, NY
The Halachah is that one who finds money with no Siman in a public place may keep it. (Some have the practice to be Machmir and put up signs (the modern day equivalent of "Hachrazah"), Lifnim mi'Shuras ha'Din, to try to return it, but this is not Halachicly necessary.)
Regarding the Rema, Rav Yisroel Pesach Feinhandler (author of Sheilos u'Teshevos Avney Yashpeh) replies:
The second source you mention is 259:7 (and not 229:7). It is clear from the Rema and also from the Shulchan Aruch 259:5 that there is a Mitzvah to return it when it is clear who lost it , even when there is not an obligation per se. Thus, if there is a clear case by Ma'os Mefuzarim the same Din would apply.