The impeachment inquiry threatens Trump's presidency even as he seeks re-election in 2020.
Gordon Sondland, who donated $1 million to the Republican president's inauguration committee, will meet behind closed doors with staff of three Democratic-led House committees.
"Simply put, he or she has done what our law demands; now he or she deserves our protection."
Lawyer Mark Zaid said the person has first-hand knowledge of some of the allegations involving the first whistleblower complaint, which triggered impeachment proceedings against Trump.
US President Donald Trump says, "Read the transcript!"
The five have been scheduled to give depositions this week and next as the Democratic-led House looks into Trump's request to Ukraine's president to investigate Joe Biden.