No but I assume he earned it legitimately, like all billionaires.
Even more legitimately than most.
Paid about 300 mill to buy a fuel company from the Russian state. Financed in part by a loan from the Russian government of approx 150 mill.
The fair market rate for the company would have been 3 billion.
10 years later he sold it back to the Russian government for 13.6 billion.