I'm siding with the Mix Engineer.
If you send your music out to be mixed, or master, all you should get back is a wav file.
How to mix a track, is a trade secret. Many producers and engineers spend years, and years practicing this. If they want to, "take their sound to grave," that's their right.
Life, liberty and all that jazz...
If you didn't work it out beforehand, and give the guy a chance to tell you, "no," beforehand.. than that is your fault.