XBOX 360 game wasn't in the sealed box when I bought it...?

I bought a game for the 360 at Best Buy tonight only to find when I got home that it wasn't in the box when I opened it. There was a blank Sony CD-R in its place. Best Buy had already closed by the time I got home and discovered the situation. Will they accept a straight exchange or did I just waste my money? It is a legitimate case of there being a blank CD in the game box, but I can certainly see how anybody could claim the same thing after taking a real game out, so I'm already a bit upset in anticipation of it being non-exchangeable. Anybody had a similar issue?