Without having looked into it at all, could there be an argument that Trump is paying a small amount of income tax because he was not actually paid a salary in that period? If his earnings come from owning companies and other assets, shouldn't he be paying capital gains tax and not income tax on that?
Again, I know nothing about the US tax system or Trump's activities in that period. Just a thought.
No, it has nothing to do with his donating his salary. He paid $0 in income tax for 10 of the last 15 years before he became president. Capital gains would be part of income tax.