He could have done it all legally but the greedy bastard didn't WANT to pay taxes!
He might have avoided a lot of taxes, with services of a good tax attorney.
Taxes need not be paid immediately on foreign source income; they can legally be deferred almost indefinitely.
I am not a tax attorney, but I heard it used to be possible to defer payment, provided the taxable net income was not "remitted" to the US parent company. In theory the funds are being used offshore, for investments etc.
A trick was however, to "lend" the funds back into a US entity. I do not know the legality of that now.
Example: US parent corporation earns profits from foreign ops. Said US parent segregates some of the foreign source taxable profit, and holds it offshore in low/no tax jurisdiction. (like Channel Islands, Bermuda, etc.)
As long as the funds remain offshore, they are not taxed in the US. The US is one of the few countries, which attempts to tax the worldwide earnings of US firms, btw.
Earn a profit on foreign project #1, keep said funds offshore, redirect said funds into another foreign project #2.
Getting US firms to remit such funds has been the subject of much political discussion, by both parties. It is said to number in the $$ Trillions.
A contact of mine, ex-US Treasury Dept. Special Agent, indicated these people may have tripped themselves up by lying about offshore accounts etc. Hence the early morning raid at Manafort's home, etc.
Investigators get some bank account statements, interview some people, get them to make statements, then trip them up for lying about having foreign money, which is a crime, even if the possession of funds is legal.
Can you say Martha Stewart? Lying to the Feds.