Tensions are running high following Donnarumma's decision not to sign a renewal once his current Milan deal expires at the end of the 2017-18 season.
The 18-year-old goalkeeper has outraged fans – who threw fake money at the shot-stopper during an Under-21 clash between Italy and Denmark on Sunday – while Mino Raiola claimed Milan bullied his client and created a hostile environment for the Real Madrid, Juventus and Manchester United target.
Fassone denied those claims and insisted there was nothing more Milan could have done to sign Donnarumma to fresh terms as the club boss gave his version of events.
"Nobody ever doubted he had the right to make this decision. It's perfectly within the rules. But it's one thing to be legitimate, another to work within the ethics of business," Fassone told Corriere della Sera.
"The bitterness here has to do with the unpleasant way the situation was handled. It could've been done without damaging the club, but by leaving like this, they have cost Milan €100m ($147 million AUD).
"If Donnarumma is worth this much at the age of 18, it's also because the club invested in him, had courage to launch him into the first team.
"All he had to do was say he didn't want to remain and we'd have renewed with a reasonable release clause. Then if the most important club in the world had turned up, even this summer, they could pay Milan that sum.
"This way what Real Madrid doesn't pay me will end up in the pockets of his agent. I at least would've reinvested that sum into Italian football.
"Every time we spoke, face to face or on the phone, Gigio assured us he wanted to stay. It was that way until two days before the last meeting with Raiola. The agent, on the other hand, always said he was not prepared to negotiate on our timescale. So there were two different versions."
Fassone also voiced his disappointment at the way business was done in football and the rift it had seemingly created between the player and the club.
"We are pained at these extremes that football seems to generate. We are also disappointed if his decision, or that of his agent, provoked such reactions.
"As a club, I really don't know what more we could've done to give Gigio and his family the desire and pleasure to remain. We tried to transmit that to him every time we spoke.
"If he were to reconsider, not only would he be welcomed back with open arms by Milan the club, but in the end I think also by the Milan fans. Moods can change so quickly.
"There were no threats. Our position is clear: Donnarumma is not for sale, and Vincenzo Montella will decide on a weekly basis. As far as I am concerned, he can play every game. However, we cannot run any risks, so we need to seek out another goalkeeper.
"We can't rely on someone who is coming to the end of his contract, who is perhaps thinking of Real Madrid, especially in such a delicate role as goalkeeper. I have to be sure we've got a focused shot-stopper who is in ideal physical and psychological condition.
"For example, if San Siro were to protest during a game, that could take something away from his concentration. From what Raiola says, the lad seems very restless."