Stam joined United for £10.6 million in 1998 but was sold to the Serie A side in 2001 after alleging that the Scot approached him illegally while he was still a PSV player in his autobiography, Head to Head.
And the Red Devils boss dashed to the defender to inform him that he was to be sold, asking him if he would agree to a swift move to Italy - with which Stam went through.
"I soon realised the club wanted to get rid of me. They also needed the money. Very soon the book became an issue again. It led to a conflict between me and the manager," he told the Sunday Mirror.
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"Ferguson said: 'Where are you?' "I said: 'Close to my house, at a petrol station.' Ferguson said: 'Wait there!' He jumped in his car and came straight to me.
"At the petrol station he parked his car and got in with me. He told me that I had to be transferred. Then he said: 'Will you please move to Lazio quickly?'
"I agreed to do it. Right there and then. One quick conversation in my car at a petrol station in Manchester was enough for me to leave that big club. When I think about it now, and I have never talked about it before, I find it unbelievable I let that, as a player, happen to me."
Despite the manner in which events unfolded at Manchester United, Stam is keen to return to England as a manager, adding: "I am getting ready to become a manager and preparing for my Uefa pro licence course. More than anything I want to go and coach in England."