The US ambassador to the EU will reportedly tell Congress next week that President Trump relayed his message there was "no quid pro quo" involved in withholding $400 million in aid to Ukraine.

Gordon Sondland will tell lawmakers he doesn't know whether Trump was being truthful when he denied the quid pro quo, The Washington Post reported on Saturday.

A September 9 text exchange between Sondland and the acting ambassador to Ukraine, William Taylor, denied the quid pro quo, and has become a key focus of the House impeachment inquiry.

The House is looking into whether Trump inappropriately pressed Ukraine to investigate one of his top political rivals, former Vice President Joe Biden.

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The US ambassador to the European Union is reportedly planning to tell Congress that his infamous text message denying a quid pro quo with Ukraine was dictated by President Donald Trump himself — and possibly not truthful.

Gordon Sondland is set to testify Thursday as part of the House impeachment inquiry. He plans to tell lawmakers he doesn't know whether Trump was being truthful when he denied a quid pro quo, The Washington Post reported Saturday, citing a person familiar with his prepared testimony.

The House is looking into whether Trump inappropriately pressed Ukraine to investigate one of his top political rivals, former Vice President Joe Biden.

Read more: Federal prosecutors are investigating whether Rudy Giuliani violated foreign lobbying laws in Ukraine

One facet of the investigation is Trump's withholding of $400 million in aid to Ukraine, and whether he sought to exchange it for the investigation into Biden.

A September 9 text exchange between Sondland and the acting ambassador to Ukraine, William Taylor, has become a key focus of the House impeachment inquiry.

In this July 10, 2018, file photo, President Donald Trump is joined by Gordon Sondland, the U.S. ambassador to the European Union, second from right, as he arrives at Melsbroek Air Base, in Brussels, Belgium. Associated Press

"I think it's crazy to withhold security assistance for help with a political campaign," Taylor texted Sondland.

Roughly five hours later, after reportedly phoning Trump, Sondland responded to Taylor.

"The President has been crystal clear: no quid pro quo's of any kind," Sondland wrote. "The President is trying to evaluate whether Ukraine is truly going to adopt the transparency and reforms that President Zelenskiy promised during his campaign."

Read more: Familiarity with the Ukraine scandal is rising and so is support for impeaching Trump

Trump and his allies have used Sondland's text as evidence that Trump wasn't trying to withhold the aid for his own personal gain.

But Sondland plans to tell Congress that he does not know whether Trump was being truthful when he denied the quid pro quo.

"It's only true that the president said it, not that it was the truth," the person familiar with Sondland's testimony told The Post.