I always understood that, when one sees a banknote in the street, one is allowed to keep it (and others wrote this way, e.g., R Belsky here and businesshalacha there #17). Then I saw the Rambam writes in Hilchot Gezelah va'Avedah 14:5 When the owner of an article does not know of its loss, h...

Although I heard in a shiur that there is a gemara which says a husband is not allowed to tell his wife not to lend to others since she would eventually develop a bad name, I can't find it - are there any sources in Chazal that say this?