The documents, including a handwritten draft of King's first anti-Vietnam war speech in 1967, had a collective pre-sale estimate of $750,000 to $1.3 million.
"The King estate believes the documents being offered in Thursday's auction are a part of the wrongly acquired collection," Isaac Farris, CEO of the King Center in Atlanta, said Wednesday. "The King estate is currently in conversations with Sotheby's to establish the truth."
Belafonte could not be reached for comment. He earlier told The Associated Press that the papers were given to him by King or his wife after the civil rights leader was assassinated in 1968.
The King estate said unnamed members of the singer's family previously tried to "anonymously and secretly" sell other such documents through a Beverly Hills, Calif., auction house. It said the estate managed to block that sale and the documents were returned to it, with an apology by the would-be sellers to Coretta Scott King. It did not cite a date for that incident.
In a telephone interview Sunday, Belafonte said he was putting the documents up for sale because "I am at the end of my life - I will be 82 shortly - and there are a lot of causes I believe in for which resources are not available, and there is a need to redistribute those resources."
He recalled how he became a close friend and early follower of King's civil rights movement in the mid-1950s and provided him with an apartment for his use on visits to New York City.
It was there, Belafonte said, that King drafted the first speech attacking U.S. involvement in Vietnam. When he flew to Los Angeles to deliver the speech to a celebrity-studded audience, he left behind the outline, written on three pages of yellow legal pad.
Also up for sale were scribbled notes for a speech King intended to deliver in Memphis, Tenn., on April 7, 1968, defending the right of city sanitation workers to strike for a living wage.
The notes, found in King's pocket after he was gunned down on April 4, 1968, on a Memphis motel balcony, were given by his wife to the late Stan Levison, a close friend who then gave them to Belafonte, he said.
The third item was a condolence letter from then-President Lyndon B. Johnson to Mrs. King, expressing sympathy over her husband's murder and promising all federal and local law enforcement resources to find the killer. Belafonte said she had given him the letter.
Selby Kiffer, a senior curator of documents at Sotheby's, said the anti-war letter would probably rank in importance with the most significant papers in King's archive, his "Letter from a Birmingham Jail," the draft of his "I Have a Dream" speech and his Nobel Peace Prize acceptance speech.