A business associate of President Trump promised in 2015 to engineer a real estate deal with the aid of the president of Russia, Vladimir V. Putin, that he said would help Mr. Trump win the presidency.

The business associate, Felix Sater, wrote a series of emails to Mr. Trump’s lawyer, Michael Cohen, in which he boasted about his ties to Mr. Putin and predicted that building a Trump Tower in Moscow would be a political boon to Mr. Trump’s candidacy.

“Our boy can become president of the USA and we can engineer it,” Mr. Sater wrote in an email. “I will get all of Putins team to buy in on this, I will manage this process.”