>>Mike- for the sake of argument- what is your friend's opinion on why it is unethical?

I don't know what Mike's friend is thinking, but the only situation I can think of where this could be referred to as unethical would be if Mike and School both deliberately inflated the value of Mike's work to bump up the employer contribution . . . . imagine the following scenario

School: Gee Mike we really enjoyed your presentation, would you come back next year and do it. We can pay you the going rate of $250 for a days presentation.

Mike: Well that sounds good, but I don't really want any money for this. I'll tell you what pay $5,000 to come and I'll donate it right back to you plus my employer has a program where they will double my donation, so you will end up ahead $10,000.

That is about the only situation here where I think an ethical issue is raised.