Yes, I agree that battlefield pickups from such battle as the Seven Days or 2nd Manassas, where large Union Supply dumps and trains were overrun and captured, for use by the Richmond depot, would be a likely source for McQ's button to have appeared on a Confederate Uniform.

The picture of the buttons in question that McQ forwarded to me, is the same as one shown on the Federal Equipments page, identified as a rare Union General Officers button, on the website URL that you attached to your late post.

Which to me makes the story even more interesting as to why McQ's ancestor kept those particular buttons through the years.