Alessio Romagnoli has been released from Italy under-21 duty to allow him to undergo a medical with AC Milan ahead of his move from Roma.
The two Serie A clubs reached an agreement on a fee -- reported to be €25 million euros plus bonuses -- on Sunday evening.
Milan had previously seen a bid of €25m rejected by Roma, who insisted on receiving €30m for the central defender.
Milan's general manager Adriano Galliani said that bid was "unmodifiable" but, in the wake of his side's disappointing showing at the Audi Cup in Munich last week, he has instead returned with a better offer.
After a meeting with Roma's sporting director Walter Sabatini on Sunday night, an agreement was finally reached.
"We're all happy," Galliani told La Gazzetta dello Sport after the meeting in Forte Dei Marmi. "It's 99.8 percent done."
Sabatini added: "We have an agreement with Milan for Romagnoli, and the deal will be formalised [on Monday]. We will ask the under-21s to release him. I've already said farewell to Romagnoli and told him he is to report to Milan."
The Italian Football Association (FIGC) announced on its official website on Monday that it had released Romagnoli "to undergo a medical pertaining to his transfer" as the rest of the squad flew out to Hungary for a friendly in Telki on Wednesday.
Should the 20-year-old successfully pass his medical, Roma will make an announcement to the Italian stock exchange confirming the transfer details.
It is the second deal done between the two clubs this summer, with Andrea Bertolacci also heading north for €20m.
Romagnoli's arrival will take Milan's summer spending to €83m -- including the €30m for Carlos Bacca and €8m for Luiz Adriano.
The Rossoneri finished 10th in Serie A last season, their second-lowest finish since returning to Serie A just over 30 years ago.