after documenting your find (and probably consuliting with an attorney) you should formally turn it in to police. The law in NY says that if it is not claimed in xx days (I forget how many) by someone who can demonstrate ownership then you get to keep it (or give it to your church).
On the off chance that it is from some ill gotten enterprise and you elect to post the notice you mention NEVER put your number on such a notice. Folks can track that back.
Lots of folks.
Including any nominal "bad guys" whose wad of cash it might be.
Or any other "bad guys" who want that wad of cash, theirs or not.
Keep What? Cash?
Municipals deposit such found/confiscated money as noted may times here in FR over the years.
You would have to petition (sue) the municipal for payment (city check or draft of check) and add a few IRS forms.