Robinho scored 25 goals in 108 appearances for the Rossoneri, but netted just four times for the Serie A club last season.
Selling the 30-year-old will open up Milan's transfer funds, allowing them to pursue their top targets. After having their interest in Arsenal's Costa Rican striker Joel Campbell rebuffed, they will turn their attention to Napoli's Goran Pandev.
Robinho's time in Milan has been something of a microcosm of his entire career. He made a big impact soon after joining in 2010 and won the Scudetto in his first season, but eventually his performance declined and he fell out of favour.
The Brazilian had a similar experience in the Premier League. He joined Manchester City from Real Madrid in 2008 as the first big-money arrival at Eastlands under the club's Abu Dhabi owners.
After scoring 15 goals in an impressive first season in England, he netted just once in 2009-10 and was soon his way out of Manchester.