For a reason unknown to me, often the ranks used for officers in the 1815 campaign use the Royal ranks from the preceeding, 1814-1815, Bourbon army. Perhaps someone can provide us an adequite explaination as to why this is so! You are correct, the rank would be major.

For others interested in this topic, see

Denys de Champaux's article "Les Souvenirs général baron de Salle" found on pp 407-437 of the La Revue de Paris, published by Bureau de la Revue de Paris, Annee 2 (1895), vol. 1, of the 15 fevrier issue:

http://books.google.com/books?id=fs8oAAAAYAAJ&pg=PA407

The quote is found on p. 430:

http://books.google.com/books?id=fs8oAAAAYAAJ&pg=PA430