President Donald Trump could be facing yet another legal headache. A watchdog group is pointing to a discrepancy between his financial disclosures and the payments made to his former lawyer as detailed in court documents.

In May, Trump signed a financial disclosure form that stated he "fully reimbursed" personal attorney Michael Cohen between $100,001 and $250,000 in 2017. But court documents filed by federal prosecutors Tuesday stated that Cohen received $420,000 from the Trump Organization over the course of last year.

The discrepancy prompted the Project on Government Oversight, or POGO, to send a letter to the Office of Government Ethics, or OGE, on Thursday, asking it to review the matter. POGO is a nonpartisan independent watchdog that investigates and exposes waste, corruption and abuse of power, according to its website.

Specifically, the letter says OGE should look at whether Trump underreported "knowingly and willfully, in violation of federal law."

It also asked for this inconsistency to expand the Department of Justice's investigation into Trump's previous financial disclosure report, which did not include any debts or payments to Cohen.

"It is quite notable," said POGO's general counsel, Scott Amey. "This may constitute a false statement by the president. If they were paying him $420 [thousand] they should have put the whole amount in there."

The OGE states on its website that criminal action could be taken against any individual who "knowingly and willfully falsifies information required to be reported." The agency reiterated the point in a tweet Thursday.

OGE tweet

John Coffee, a professor at Columbia Law school focusing on securities regulation and white-collar crime, said the difference of $170,000 is quite likely material, "and thus, a potential criminal violation."

The whole situation revolves around payments that Cohen, Trump's former personal lawyer, made to adult film star Stephanie Clifford, also known as Stormy Daniels. Cohen is known to have paid Clifford $130,000 just before the 2016 presidential election in exchange for her silence over an alleged affair with Trump.

Trump and his team initially dodged questions on the topic, and Trump denied knowing anything about the payment in April. But Rudy Giuliani, the president's lead outside counsel in the Russia probe, confirmed the payment in early May in an interview with Fox News. Trump then tweeted about it, saying it wasn't campaign related.

TRUMP tweet

Ethics experts wondered whether Trump's debt to Cohen would be disclosed on his OGE form 278e, a financial disclosure that candidates and public officials in the executive branch are required to complete annually.

Sure enough, the liability was reported in a footnote, dated May 15, 2018, and signed by Trump. The OGE told the Department of Justice that the payment made by Cohen was required to be reported as a liability and that the information provided in the form fulfilled that requirement.

But now, with Cohen pleading guilty to the felony of making an excessive campaign contribution, and implicating the president in the process, the value of the disclosure is getting a fresh look.

The filing states that Cohen showed a copy of a bank statement to executives of the Trump Organization, reflecting the $130,000 he paid Clifford. He also added in a wire fee and another $50,000 for "tech services."

According to the filing, it was "the Company" that "grossed up" the requested reimbursement for "tax purposes" – to make sure Cohen got paid in full after paying taxes – an unorthodox practice for a reimbursement. They also added a bonus of $60,000. Cohen then sent monthly invoices, and the total of $420,000 was paid to Cohen in $35,000 increments.

Amey says he doesn't think there's much "wiggle room" over the value of the disclosure.

By the time the president signed the form on May 15, 2018, the full sum has been paid out to Cohen.

The concern over the discrepancy is not unanimous. Since U.S. attorneys for the Southern District of New York viewed Cohen's payment to Clifford as an illegal campaign contribution, it may become unnecessary for Trump to disclose it personally.

But Trump can't have it both ways. If he claims the payment had nothing to do with the campaign, as he did in his Fox News interview this week, then it would need to be disclosed on his form 278e. Walter Shaub, former director of the OGE, and former Obama ethics counsel Norm Eisen each tweeted about the issue this week. They are both frequent critics of Trump.

Schaub tweet

Eisen tweet

To be clear, Amey says he is unaware of anyone being charged for an inaccuracy on a financial disclosure like this. And Coffee points out, "You can make a factually incorrect statement without it being criminal," for example, if Trump did not know about the additional payments beyond $130,000.

The White house did not return CNBC's request for comment by the time of publication. The Office of Government Ethics pointed CNBC toward the letter it sent the Department of Justice in May and had no comment on the current matter.