The accusation, according to the source, is related to Trump's alleged repayment of money to Cohen for a hush money deal with Stormy Daniels, the adult film actress and director who claims to have had an affair with Trump. Trump has denied Daniels' claim but has admitted to the reimbursement.
Cohen, Trump's former personal attorney and longtime fixer, will also provide the House of Representatives' Committee on Oversight and Reform with detailed documents that will show the president allegedly inflated and deflated his assets and net worth for personal and professional purposes, including to reduce property taxes.
He also plans to share personal anecdotes and eyewitness accounts about Trump, including alleged instances of racism, lying and cheating.
Cohen is additionally expected to explain his motivation for previously lying to Congress in 2017 in order to protect Trump, the source told ABC News.
Cohen's highly-anticipated testimony before the House committee comes after he postponed his initial appearance due to "threats" to his family by Trump and the president's attorney, Rudy Giuliani.
Cohen is set to begin a 3-year sentence in federal prison in early May for an array of crimes he pleaded guilty to committing while working for Trump.