He said: "This decision rests with the club but I can only stress that Kaka is very important to us and that his exit would be hugely negative.
"For me this deal sounds incredible. No player is worth this much, not even Cristiano Ronaldo. But Ricky [Kaka] has said nothing about this to us. He seems pretty calm about everything."
Kaka is expected to feature for Milan at home to Fiorentina on Saturday night and Maldini warned City that they will have to offer his team-mate more than just money if they are to persuade him to leave.
Maldini added: "I have known him for years and I am convinced he won't have his head turned just by the money. He is not like that."