The five-million euro clause due to Real Madrid, Cassano's club from 2006-2007, which had been holding up the deal, will be jointly paid by Sampdoria, AC Milan and the player himself, according to Gazzetta dello sport.
The contract will not be filed before the official opening of the transfer market on January 2, but agreement has been reached allowing the player to rejoin his new team's winter training camp in Dubai on December 27.
Cassano's arrival could herald the exit of Brazilian Ronaldinho, whose appearances for the team have become increasingly rare and could be sold to Los Angeles Galaxy.
Cassano, 28, joined Sampdoria from Real in 2007. But he has been frozen out of the club since insulting president Riccardo Garrone during an argument in October.
Sampdoria had applied to have his contract unilaterally terminated for gross misconduct, but an Italian league arbitration board ruled last week that his salary should be cut by 50 percent, but that his contract could not be cancelled.
Cassano has 20 international caps and had looked set to become a first-choice player under new Italy coach Cesare Prandelli.
The former Real Madrid and Roma star has attracted controversy throughout his career for on-pitch tantrums and disputes off it with coaches, presidents and team-mates