Sampdoria claimed that Pazzini had fouled Da Costa and the hosts were even more incensed shortly afterwards when striker Eder was upended by Milan goalkeeper Amelia in the penalty area.
The referee judged that Amelia got the ball before making contact with Eder, although television replays were inconclusive.
Sampdoria substitute Maxi Lopez was given two yellow cards in quick succession amid the protests and was sent off after only 16 minutes on the pitch.
Fifth-placed Inter (40 points) needed a second half equaliser from Rolando for a share of the spoils after Chile forward Mauricio Pinilla had put the Sardinians in front with a penalty five minutes before halftime
Bosko Jankovic, Romulo and Luca Toni scored in a 12-minute spell before halftime for sixth-placed Verona (39) at Livorno, who replied through Paulinho and Leandro Greco midway through the second half.
Antonio Di Natale's second-half penalty gave Udinese a 1-1 draw at home to Atalanta and Chievo beat fellow strugglers Catania 2-0.)