What should a husband do when he finds 40 Euros on a busy road in France and does not know its owner?

This money is a lost item (Luqata), to be publicized for one year. After that, the finder may dispose of it, according to the Hadith of Zayd ibn Khalid Al-Juhani. When publicizing it, its type or nature should not be mentioned. Rather, it is sufficient to say: "O whoever has lost something." Your husband can write the phrase "O whoever has lost something, please call this number" and paste it near the place where it was found.