Two years ago, Andrew Intrater’s Manhattan private equity firm was thriving. He had rubbed elbows with billionaires at President Trump’s inauguration. And he had a lucrative investment involving the heirs to the fortune of the musician Prince.

Today, the firm is effectively shuttered, and Mr. Intrater cannot get access to his money from the Prince investment or just about any other deal. Wall Street cut ties with him. For months, his only checking account was with a small agricultural bank 1,200 miles from New York. Even his T.S.A. PreCheck privileges were revoked.

What had changed? For starters, the United States sanctions against Russia had swept up businesses with links to Russian oligarchs. Mr. Intrater and his firm are American and not directly subject to the penalties, but his cousin and biggest investor is the oligarch Viktor Vekselberg, who was hit with sanctions in April last year.

Mr. Intrater’s problems worsened soon afterward, when leaked financial documents tied him to Michael D. Cohen, a former lawyer for President Trump who was under investigation and is now in prison. Mr. Cohen’s critics suggested that Mr. Intrater may have been a front for his oligarch cousin to funnel Russian money to the president’s inner circle, a theory Mr. Intrater denied. The special counsel’s office, which interviewed Mr. Intrater about the issue, never accused him of any wrongdoing and did not mention him in its report.