Milan have made public their interest in Belotti, who has a release clause of €100 million for foreign clubs after scoring 26 goals in Serie A last season.
Torino have no plans to sell the 23-year-old and nine-time Italy international, as long as he remains motivated at Stadio Olimpico Grande Torino.
"The lad's done very well with us but he has to stay with us and he can stay," Cairo told Premier Sport. "I'll only be forced to sell him if someone abroad comes forward with the player's release clause to buy him.
"I haven't negotiated with anyone. I've always said that I'd like to have him motivated here, but I've also said that in all my companies, I have motivated staff.
"He is engaged with us until 2021. And I think he would do well to stay one more year with us, to go to the World Cup with momentum. He can have a great season with us.
"I don't want to sell him, but there's a clause. If someone from abroad comes with €100m, then I shake his hand and wish him good luck."
Milan have completely overhauled their squad, signing Leonardo Bonucci, Andre Silva, Hakan Calhanoglu, Lucas Biglia, Andrea Conti, Ricardo Rodriguez, Mateo Musacchio, Franck Kessie and Fabio Borini, and former striker Vieri wants Belotti to join the revolution at the San Siro.
"A new striker for Milan? [Nikola] Kalinic has more quality with the ball at feet, but I prefer Belotti because he's bigger, hungrier and more of a handful," Vieri told Sky Sport Italia.
"Still, he must continue to fight like an animal. He reminds me of [Torino's all-time top scorer Paolo] Pulici. I advise him to go to a big team quickly. He must make another leap in quality."