AC Milan fear financial factors could ultimately make it impossible to agree any deal to take Kaka back from Real Madrid.
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."