Real Madrid coach Jose Mourinho has told AC Milan they must pay a transfer fee if they want Kaka to return to the San Siro.
Mourinho was speaking after Madrid had hammered the Serie A side 5-1 in a friendly at New York's Yankee Stadium on Wednesday evening.
"I would be happy if [Kaka] stays with us or is sold - what I want is that he himself is happy," Mourinho said. "It is always said in football that a player plays where he wants. My arms are open if he wants to continue - against Milan he showed also that he is a great player and can compete.
"But nobody goes for free to these places. If you want something, you have to pay for it."
Kaka, 30, appeared as a substitute for the final half hour of Wednesday night's game, laying on Madrid's third goal for Cristiano Ronaldo and then taking the corner from which Sergio Ramos headed the fourth.
Milan coach Massimiliano Allegri said that it was possible the Brazil playmaker could return to the Rossoneri, where he won Serie A, Champions League and FIFA Club World Championship titles between 2003 and 2009.
"That is a decision for the board, not for me... we will see what happens in the future," Allegri said. "The transfer window is open until August 31 and anything is possible."
Spanish daily Marca reported on Thursday that respective club presidents Florentino Perez and Adriano Galliani had discussed a possible transfer when they spoke before the game.
Milan's Galliani is believed to want Kaka on loan for the coming season, with the two clubs sharing his €9 million a year salary, while Madrid's Perez would prefer a straight sale. A figure of €25m has been mentioned.
Galliani has told Sky Italia: "The operation for Kaka is objectively very, very difficult. In fact it is almost impossible. Costs are high and his salary is very high."
Since his move to Madrid for €65m three summers ago, Kaka has struggled to settle due to regular injuries and loss of form. He made just 21 La Liga and Champions League starts for Mourinho last season, and was generally a substitute in the biggest games.
The former World Footballer of the Year's most memorable contribution was missing a penalty in the shoot-out against Bayern Munich as Real exited that competition at the semi-final stage. Earlier this month, Mourinho hinted that Kaka would feature even less regularly for Madrid in the 2012-13 season.