On Thursday, Mr Trump said he would leave the White House if Mr Biden beats him in the Electoral College. On Friday, he backtracked on the pledge and appended a number of additional conditions that Mr Biden would have to meet to ensure the president's peaceful exit from the White House.
Mr Trump's tweet repeated unsubstantiated claims about voter fraud.
"Biden can only enter the White House as President if he can prove that his ridiculous “80,000,000 votes” were not fraudulently or illegally obtained. When you see what happened in Detroit, Atlanta, Philadelphia & Milwaukee, massive voter fraud, he’s got a big unsolvable problem!" Mr Trump wrote.
Biden can only enter the White House as President if he can prove that his ridiculous “80,000,000 votes” were not fraudulently or illegally obtained. When you see what happened in Detroit, Atlanta, Philadelphia & Milwaukee, massive voter fraud, he’s got a big unsolvable problem!
— Donald J. Trump (@realDonaldTrump) November 27, 2020
There is no evidence to support Mr Trump's claims.
Detroit, Atlanta, Philadelphia and Milwaukee are all major metropolitan areas, which tend to favour Democrats and have large Black communities.
His initial pledge to leave the White House was made on Thursday when he was asked by a reporter if he would go peacefully.
"Certainly I will, and you know that," Mr Trump said. "I will, and you know that."
Even then, he still repeated the unsubstantiated claims that there had been massive voter fraud.
"It's going to be a very hard thing to concede because we know there was massive fraud," he said. "As to whether or not we can get this apparatus moving quickly - because time isn't on our side, everything else is on our side, facts are on our side, this was a massive fraud. This should never take place in this country, we're like a third-world country."
He continued complaining about the election results and said that if Mr Biden is declared winner by the Electoral College, it will be a mistake "cause this election was a fraud."
When a reporter pushed back on Mr Trump's false claims, the president snapped at him, calling him a "lightweight" and complained about how he was being addressed.
"Don't talk to me that way. I'm the President of the United States. Don't ever talk to the president that way," Mr Trump said.
The press briefing was the first time Mr Trump has taken questions from reporters since the election.