President Trump said Thursday that he only hired attorney Michael Cohen because "he did me a favor" long ago — and former Trump confidant Roger Stone says he knows what that favor was.

"Cohen, whose wife is extremely wealthy, purchased a number of Trump condos at top-of-the-market prices," Stone told the Washington Examiner, explaining the president's vague remark.

The White House did not respond to a request for comment, and a person close to Cohen denied Stone's account without offering an alternative explanation.

Cohen began working at the Trump Organization in 2006 and built a reputation as the future president's "fixer," aggressively defending Trump before pleading guilty to financial and campaign finance crimes in August and agreeing to cooperate with federal prosecutors.

[Byron York: Remember Prague? In Michael Cohen plea deal, Mueller says nothing about key collusion allegation]

Before working for Trump, Cohen purchased multiple units at the billionaire developer's buildings, according to reports.

"[Cohen] has invested in my buildings because he likes to make money — and he does," Trump told the New York Post in February 2007, about a year after he hired Cohen.

The Post reported that Cohen "purchased his first Trump apartment at Trump World Tower at 845 United Nations Plaza in 2001" and "was so impressed he convinced his parents, his in-laws and a business partner to buy there, too. Cohen’s in-laws went on purchase two more units there and one at Trump Grande in Sunny Isles, Fla."

"Cohen then bought at Trump Palace at 200 E. 69th St., and Trump Park Avenue, where he currently resides," the Post reported in 2007. "He’s currently in the process of purchasing a two-bedroom unit at Trump Place on Riverside Boulevard — so, naturally, Cohen’s next step is to purchase something at Trump Plaza Jersey City. He’s now in negotiations for a two-bedroom unit there."

On Thursday, Cohen pleaded guilty to a new charge of lying to Congress about Trump's opening of a hotel in Moscow during the presidential campaign, leading to a strong denunciation from Trump, who called Cohen a liar and "not a very smart person."

"If Cohen is such a bum, why did you hire him, have him on your payroll for 12 years, and have him do so much of your dirty work, like paying off a stripper?" a reporter asked Trump on the South Lawn of the White House.

"Because a long time ago he did me a favor. A long time ago he did me a favor," Trump said. The president did not elaborate on the "favor" before embarking on a 10-hour flight to Argentina for the G-20 summit in Buenos Aires.

Trump confidant Stone has resisted cooperating with special counsel Robert Mueller, unlike Cohen.