Some of the most explosive parts of a dossier containing unverified allegations that President Donald Trump had secret ties to Russian leaders originated from the Belarus-born head of a Russian-American business group, according to a person familiar with the matter.

Sergei Millian, a 38-year-old American citizen who has claimed he helped market Trump properties to Russian buyers, wasn’t a direct source for the 35-page dossier, this person said. Rather, his statements about the Trump-Russia relationship were relayed by at least...