I would make them put the Metz on since you had them order it for you and that is what you agreed on. Don't let them give you any BS about extra charges for mounting another tire since they screwed up not you. While you could run the 2 different tires with no problem it isn't what you asked for or agreed on.

I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!