Mr Comey has been a vocal critic of Mr Trump since he was sacked by the United States president in May and the intelligence chief's explosive memoir has been heavily trailed by USA media.
Although the preview only features Stephanopoulos' line of questioning, of which Comey apparently answered each and every one, the implications alone are damning.
The source said Comey's comments, in his first interview since being fired by President Trump last May, will generate headlines and "certainly add more meat to the charges swirling around Trump".
"How unusual is it for you to sit here and compare the president to a mob boss?". Stephanopoulos asks at the start of the promo video.
Trump has defended himself, saying he simply fired Comey "very simply" because "he wasn't doing a good job".
Trump has called Comey a "leaker", accused him of running "the phony and dishonest Clinton investigation" and used him as a means to challenge the FBI and the Justice Department. Comey also says he encouraged Kelly not to quit because the president needed people like him around to help.
The former Director spoke of things that he had never said before, the source told Axios news.
In an interview with ABC News' George Stephanoplous airing on Sunday, Stephanopoulos suggests that Comey compared Mr. Trump to a "mob boss".
The memoir became a No. 1 best-seller on Amazon's top 20 books list last month, based exclusively on pre-sales.
The White House also said Trump's decision was based on the recommendations of Attorney General Jeff Sessions and Deputy Attorney General Rod Rosenstein, who at Trump's request had written a memo about Comey that was critical of the Federal Bureau of Investigation director.