The former World Player of the Year and Ballon d’Or winner first moved to Spain in a £50.7million switch in 2009 on a six-year contract.
But Kaka has struggled for form and fitness at Real and he could leave the Primera Liga champions this summer if any interested clubs meet a reported £40m asking price.
The Brazil international’s agent has previously revealed talks are taking place with Milan but the Italian club’s vice-president, Adriano Galliani, worries any transfer could prove too expensive.
With tax laws in Spain giving players lucrative contracts, Galliani told Sky Italia: “It is very difficult.
“The negotiations are all to be done. There are a thousand difficulties that could well make it impossible.
“As always, the problem is money. Milan cannot afford some wages, especially as in Spain they have a different fiscal structure.
“The law there has changed for future wages but the contracts that were already signed are still valid under the old tax code.”