The Real Madrid midfielder has struggled for form and fitness following his big-money switch to Santiago Bernabeu in 2009.
Kaka has been told he is allowed to leave the La Liga champions, provided potential suitors meet the reported £40 million asking price.
The midfielder's agent has revealed that talks over a potential return had taken place.
However, club vice president Adriano Galliani believes that any transfer could be too expensive, with tax laws in Spain giving players lucrative contracts.
"It is very difficult," Galliani told Sky Italia. "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."
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