My understanding is that you can sell stock (that might pose a conflict of interest) tax-free when you take a government role. I presume that a hedge fund manager such as James Comey would have significant unrealized capital gains that he could sell prior to taking his government position.

Gary Cohn, Trump's chief economic advisor did exactly this[0].

"The chief economic adviser was required to sell his Goldman Sachs shares -- and he did so tax-free and with perfect timing"

[0] https://www.marketwatch.com/story/a-rich-gary-cohn-got-even-...