Sir Alex Ferguson considered signing Sergio Aguero for Manchester United in 2011 but backed out because the player's agent "was demanding a price we were not prepared to pay."
Instead, Aguero joined Manchester City for £38 million and scored the goal that cost United the Premier League title the following season.
Ferguson's United had been set to become the 2011-12 Premier League champions until the Argentine scored a 94th-minute winner for City against QPR.
Aguero has gone on to score 108 goals in 171 games for City, and won the Premier League Golden Boot last season.
Ferguson, explaining why the United move never happened, wrote in his new book "Leading": "His agent was demanding a price we were not prepared to pay."
The Scot also revealed that he had also weighed up the possibility of signing Mario Balotelli, another striker who went on to play for City.
But he decided it would have been too great a gamble to bring in the unpredictable Italian after consulting others.
"I did my homework on him, speaking to a few Italian contacts, but the feedback I got confirmed it was too big a risk," he wrote.
"I don't know whether this sort of candid professional courtesy exists in other fields, but for me it was a Godsend."
Balotelli is currently on loan at AC Milan after an unsuccessful year at Liverpool.
During his spell at City, he scored twice at Old Trafford when Ferguson's United suffered a 6-1 loss -- the manager's heaviest ever defeat.