Given the Bus/Financial context, a 'chargé de mission' is someone who is hired for a specific job within an existing structure, or someone from within an existing team, who is assigned repsponsibilité for a given project.

This is usually a fixed-duration mission, rather than a permanent job (often a secondment from another position internally).

As a 'chargé de mission' is usually quite seperate from the general company hierarchy, I would avoid using the term 'manager'. Usually the person is responsible for a specific task rather than a team of people.

Hope this helps