AC Milan vice-president Adriano Galliani has admitted that the club's move for Real Madrid's Kaka is "very difficult".
The Brazilian playmaker is believed to be in talks with the Serie A giants, whom he left in 2009.
However, Galliani has revealed that discussions with Kaka, whose old number 22 shirt has been left unassigned, have ground to a halt over disagreements with Real Madrid over money.
"It will be very difficult," Galliani told reporters. "There are a thousand factors which could make this impossible.
"As always, the problem is money. Milan cannot afford to pay some wages, because Spain have a different tax system.
"The law there has changed but only for future contracts. The contracts that were already signed will stay as they were."
Kaka has made 92 appearances for Real Madrid, scoring 24 goals.
