The seven-time European champions remain in fifth, behind fourth-placed Lazio on goal difference in the race for Champions League qualification.
Milan have said they will appeal to the Court of Arbitration for Sport after UEFA threatened to ban them from European competition for a season if they do not break even by June 2021.
The club won a previous appeal to CAS earlier this year, following a Europa League ban for breaching Financial Fair Play regulations.
But their efforts to secure European football for next season suffered a setback against a resolute Frosinone outfit who almost snatched a dramatic 93rd-minute winner when Camillo Ciano was denied by visiting goalkeeper Gianluigi Donnarumma.
Ciano had already seen a goal ruled out by VAR for a foul eight minutes before half-time.