It is important to understand that engagement is not Nikkah; it is merely a promise to get married. Until the actual Nikkah takes place, the girl and boy are still considered non-mahrams to each other. Therefore, it is not permissible for the boy to place a ring on the girl's finger during the engagement ceremony, as this would involve touching a non-mahram, which is considered a serious sin. According to a Hadith, "It is better for one to injure oneself with an iron needle than to touch a non-mahram woman." (Mou'jamul kabeer)