The midfielder walked off the pitch during Milan's friendly against Pro Patria in June because of racist abuse from fans, and Schalke's Peter Peters revealed that such incidents left him wanting to play his football elsewhere.
"Because of the racist incidents, Boateng wanted to leave Italy," Peters told SportBildPlus.
"He had an arrangement with president Silvio Berlusconi that he could leave if AC Milan qualified for the Champions League group stage."
Both Milan and Schalke made it through to the group stages of Europe's elite competition, with Boateng also helping Schalke to their first Bundesliga win of the season on his debut.
The 26-year-old played 84 minutes before being replaced by Christian Clemens as Jens Keller's men beat Leverkusen 2-0 on August 31.
Boateng signed a four-year deal at Schalke the day before making his first appearance for them.
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