Keane was given a £5,000 fine after criticising United's performance following a 4-1 loss to Middlesbrough in 2005 and it was his decision to appeal the fine which led to his departure.
It is widely believed that the Irishman left the club due to clashes with Sir Alex Ferguson but Keane insists this is not the case.
"The only reason I ended up leaving Man United was I appealed a £5,000 fine. If I hadn't appealed the fine I wouldn't have left Man United," Keane told the Limerick Leader.
"Don't be brainwashed by the propaganda and lies. It is peanuts, you get fined that for being late for training at United.
"I appealed because I felt I shouldn't have been fined for speaking about a match on the club station."
Keane criticised a number of United's players following their poor display at the Riverside and the interview was deemed too extreme to air on the in-house TV channel. The midfielder ended up leaving United my mutual consent just over two weeks after the Middlesbrough defeat in November 2005.
"Have I stepped out of line? Damn right I have," he added. "Man United fined me about £500,000 over my career but that is because I was sent off and I always held my hands up.
"Ironically when I appealed a £5,000 fine that's when I left the club. I wished they played this video, it is propaganda, "we had to destroy it".
"How do you even destroy a video? I left with my head held up high, I was fine with my actions, I always felt my intentions were to do the best for Man United."