Is anyone aware of a way that I can prove that I used MM miles to upgrade a flight?

Basic situation is that an organization has invited me for a presentation and agreed to reimburse me for a flight in economy class. When they asked for my boarding passes the administrator had a heart attack because it says business on the BP, because I used my MM miles to upgrade one of the legs.

Does anyone have any idea how I might best be able to prove to this administrator that I used my own miles for the upgrade and didn't surreptitiously buy a business class ticket (which would have been a fantastic bargain had it been possible for the price on my ticket receipt).

Many thanks!