Washington (CNN) The US ambassador to the European Union, Gordon Sondland, is expected to tell Congress this week that President Donald Trump relayed to him directly in a phone call the content of a text message that Sondland sent denying quid pro quo, The Washington Post reported Saturday, citing a person familiar with his testimony.

Sondland's text message was sent to the top US diplomat in Ukraine, Bill Taylor, who raised concerns in a text to Sondland about the US withholding nearly $400 million of US military and security aid. This text message exchange has become a major focal point of the impeachment inquiry into the President.

Sondland is expected to testify to Congress that he has no knowledge of whether Trump was telling him the truth at the time, the Post reports. "It's only true that the President said it, not that it was the truth," the person familiar with Sondland's planned testimony, who spoke on condition of anonymity to discuss the sensitive diplomatic matters, told the Post.

The congressional testimony by Sondland , a key witness within the State Department and to the President's action in the ongoing impeachment inquiry, could prove damaging to the President and his personal lawyer, Rudy Giuliani, the Post reports. Sondland intends to testify to the House under subpoena Thursday, according to his lawyers.

Sondland is a Trump donor and hotelier who has been EU ambassador since 2018. He exchanged messages with former US special envoy to Ukraine Kurt Volker and a senior US diplomat in Ukraine about setting up the call between Ukrainian President Volodymyr Zelensky and Trump. He also exchanged messages about whether foreign aid was being withheld while Trump and Giuliani pushed for Ukraine to open an investigation into business activity by the son of one of the President's 2020 Democratic rivals, former Vice President Joe Biden.