Genoa have revealed the pricey financial details that will see Milan sign a half share in Stephan El Shaarawy on Friday.
Genoa chief Enrico Preziosi had previously insisted that the 18-year-old would not be sold this summer, but Milan's massive offer has seen him change his mind.
"Their bid surprised me," he told Il Corriere Mercantile. "I really didn't expect it at all, otherwise I wouldn't have made those statements last week.
"The Rossoneri valued him at €20m. So that is €10m for a half share in him now, but the value of his other half can increase too if he does well.
"Overall, it's a deal worth €25m and I just couldn't say no."
As part of the negotiations, Genoa will sign a half share in Rossoneri youngster Alexander Merkel in return.
"I've read that we will pay €1.3m for Merkel, but that's not true," added Preziosi. "He'll cost us €5m because Milan value him at €10m.
"He is very young, but he is also ready for our championship."
El Shaarawy, an attacking midfielder, has only played three games in Serie A for Genoa and 27 in Serie B while on loan at Padova this season.
Midfielder Merkel, meanwhile, is just 19 and he has made only six top-flight appearances for the Rossoneri. They all arrived this term.
"I understand the disappointment of the fans that we are losing a youngster like El Shaarawy, but I have to put the good of Genoa first," Preziosi added.
"The kind of offer Milan made was one that you have to accept in an instant."