The veteran broadcaster, who has chaired BT events for the past three years, turned down the offer because the interviews would have been used to promote the company - something forbidden under BBC rules.
But the offer illustrates the amounts of money available to the corporation's top broadcasters. Agents who act for them say that fees range from pounds 2,000 to pounds 5,000 a day for taking part in internal company videos, staging question- and-answer sessions or chairing conferences.
However, the size of the BT offer is understood to have made Mr Humphrys wary, according to a source who was involved in the negotiations. He was initially told that the quarterly interviews with Sir Iain would be for internal use only, which could not be construed as promoting the company.
A short time into the discussions, it emerged that stills from the interviews may have been used in the firm's advertising and so Mr Humphrys turned down the offer.
The apparent contradiction in the BBC's Producer Guidelines, which lay down strict rules for public consumption on what freelance workers like Mr Humphrys may do outside BBC hours, and the freelance contracts it allows them to sign, is the reason for concern among senior executives at the corporation.
Decisions on what is and is not acceptable are left to the broadcasters involved. Only their integrity stands in the way of the system being abused.
Mr Humphrys said it was "inconceivable" that accepting money from large companies or Government departments could ever influence him.
"In 35 years as a journalist, no one has ever accused me of anything of the sort," he said. "Everything I do is completely above board. As freelances, we are not allowed to broadcast for anyone else, but we are allowed to do work that does not undermine our integrity or the integrity of the BBC, and that precludes promotion and advertising."
Mr Humphrys is meticulous in his application of the rules and will only work for a company if his involvement in no way promotes it to the outside world.
"When I consider accepting work, I ask myself whether I would be absolutely satisfied with it being broadcast on the Today programme or on television and if I would, then I will accept it," he said. "If people try to script what I do or give me questions to ask, then I say forget it."
Most of the BBC's freelance presenters have done outside work at some time or another but, according to Mr Humphrys, a "hell of a lot is turned down as being unsuitable".
He confirmed that he had carried out media training for ICI several years ago, but said that recently he had only been involved in training charities.
"If the BBC were to bring in a register, not necessarily like a book that MPs fill in, but a register in as much as a requirement to let your head of department know what you are doing, I would have no problems at all with that," he said.Reuse content