In Victor Hugo's Les Misérable, why does Jean Valjean tell Marius' grandfather, M. Gillenormand, that Cosette's real name is Euphrasie?
Volume V, Book V, Chapter IV: "Mademoiselle Gillenormand Ends by No Longer Thinking It a BAd Thing"
Euphrasie Fauchelevent is actually Cosette's full, legal name. We learn this in Volume 1, Book IV, Chapter I: "One Mother meets Another Mother." In this chapter Fantine has already been abandoned with child by Tholomyes and has traveled on foot to Montfermeil and found herself near the Thenardier hostelry. The narrator then spends a great deal of time describing Madame Thenardier's beautiful and beautifully dressed daughters and describes the blue eyes of the child in Fantine's arms. When Fantine introduces Cosette to Madame Thenardier we learn that Cosette's real chosen name is Euphrasie. The narrator states: "For Cosette, read Euphrasie. The child's name was Euphrasie. But out of Euphrasie the mother had made Cosette" and further explains that Fantine changed the name in the same way that "Josepha [changes] into Pepita, and Francoise into Sillette." In other words, Cosette is a nickname, much in the same way that today Jack is a common nickname for John, as it was for John F. Kennedy, and Dick is a common nickname for Richard, as it was for Richard Nixon.
Therefore, when later in the novel Jean Valjean negotiates Cosette's dowry with Marius' grandfather, M. Gillenormand, Valjean informs him that Mademoiselle Euphrasie Fauchelevent has six hundred thousand francs (Volume V, Book V, Chapter IV: "Mademoiselle Gillenormand Ends by No Longer Thinking It a BAd Thing"). Euphrasie is Cosette's full, legal name.