President Trump's brother secured a court order Tuesday temporarily blocking the publication of a book by his and the President's niece.
Mary Trump's upcoming book, Too Much and Never Enough, How My Family Created the World's Most Dangerous Man, is due to be published on 28 July.
The complaint claims that the only exception that would allow Mary Trump to reveal such information would be if the president, Robert Trump, and their sister, Judge Maryanne Trump Barry consented.
Using that background, she explores the "nightmare of traumas" within the Trump family, according to the publisher.
Greenwald's order blocks the distribution of Mary Trump's book until he decides whether to issue a formal injunction.
Robert Trump argues Mary Trump must comply with a written agreement among family members who settled a dispute over Fred Trump's will that a book about them can not be published without their permission.
In a statement from Mary Trump's lawyer, Mr Boutrous said: "President Trump and his siblings are seeking to suppress a book that will discuss matters of utmost public importance".
"We plan to immediately appeal this decision to the Appellate Division, and look forward to prevailing in this case based on well-established precedents regarding prior restraint", the publisher said, according to the news outlet.
Charles Harder, an attorney for Robert Trump, said his client was "very pleased".
Mary Trump's lawyer Ted Boutrous has indicated she will "immediately" appeal the order, even though it is only temporary.
"We look forward to vigorously litigating this case and will seek the maximum remedies available by law for the enormous damages caused by Mary Trump's breach of contract and Simon & Schuster's intentional interference with that contract", Mr Harder said.
Mary Trump is a trained psychologist who was reportedly a source for New York Times reporting on Trump family tax affairs and who has expressed opposition to her uncle on Twitter. Click the link to confirm your subscription and begin receiving our newsletters.
Adam Rothberg, a spokesperson for Simon & Schuster, also expressed disappointment in the court's decision.
"You know, when we settled with her and her brother, who I do have a good relationship with - she's got a brother, Fred, who I do have a good relationship with, but when we settled, she has a total ... signed a nondisclosure", he said.
Robert Trump, the president's brother, filed a suit last week in NY in Queens County Surrogate court, where the estate of the president's father, Fred Trump Sr., was settled after his death in 1999.
Robert Trump, the president's younger brother, sued to prevent it from coming out.