Cassano is keen to quit the club with negotiations already underway about a possible swap deal with Giampaolo Pazzini of Inter Milan, who is searching for a new club with Inter seeking a different front man to partner Diego Milito.
Milan chief executive Adriano Galliani confirmed the possible exchange between the rival clubs after the Rossoneri's 3-2 defeat by Serie A champions Juventus in the Berlusconi Trophy friendly on Sunday.
"Talks are ongoing. Antonio has asked to be sold, let's see what happens. The most probable scenario is a swap," he said.
Cassano joined Milan from Sampdoria in January 2011, and Allegri said: "It’s a shame. Antonio has been here for a year and a half.
“He did really well in his first six months where he made the difference. Then the problem with his heart stopped him.
“I think that despite the qualities of Cassano, it’s not a good idea to keep players against their will – it would not be good for Cassano or Milan.
“We need a forward who can play close to a centre-forward. Let’s see how this situation evolves.”