I found a $100 bill in the vicinity without any Siman. Can I keep it, and do I need to give Tzedakah from it?
If the money was found in a place that many people pass through and was lying in a way that it appears to have fallen there (as opposed to having been placed there), e.g. it was not covered, it was without a wallet or the like, it was not in a somewhat-safe place, then it belongs to the one who found it.
If it was found in a place that does not have many people passing through, then one must assume that it belongs to the owner of that place or his family. If the owner confirms that it is not his, and it meets all the other criteria mentioned in the previous paragraph, then it belongs to the finder.
Even if these conditions are lacking, if it was found in a place where most passersby are non-Jewish, or even if most passersby are Jewish, but non-Jews sit there regularly (such as a location where there are non-Jewish guards), one may keep it. Nonetheless, in this case, it is right and proper to go beyond the letter of the law and return it, if one knows clearly who it belongs to.
Maaser from the find should be given to Tzedakah.
ראה שו”ע אדה”ז הל’ מציאה ופקדון סעי’ ח’-י”ב, שם י״ז-י״ח. וראה הנסמן בצדקה ומשפט פרק ה’ סעי’ ה’, ועוד.