Why is Felix Sater at the centre of US power?

The New York Times reported last weekend that one of Donald Trump’s lawyers, Michael Cohen, along with two other men, attempted to deliver a sealed proposal for crafting a Ukraine peace accord and lifting Russian sanctions to the then National Security Adviser Michael Flynn shortly before Flynn resigned.
Cohen disputed a portion of the Times’ account to reporters with The Washington Post, arguing that he hadn’t taken the sanctions plan to the White House nor had he spoken to anyone about it.