I agree that it's a German G 98 Mauser.
I think it is most likely a prop provided by the studio. It makes sense, if you think about it. Any soldier who wanted a commercial portrait wouldn't have their own issue rifle with them when they were "out on the town," so the studio would have rifles to use as props. The fact that it's a German rifle makes sense as well. Where would a French photographer get a US Springfield 1903, for instance, or even a French Lebel as that was the current French issue piece. But, a captured German rifle, bought locally, could be easily acquired, especially if it was after the Armistance.