Milan are reported to be pushing Villarreal to reduce the amount of a previously agreed purchase option for defender Cristian Zapata.
The Colombian returned to Serie A on loan last summer, after an unsuccessful season with the Yellow Submarine in La Liga.
The Rossoneri agreed an initial €400,000 to take the centre-back temporarily through to this coming summer, with a further €6m agreed as a fee payable in June, if the Italians wanted to make permanent the deal.
However, where Milan are seen as keen to keep hold of the 26-year-old, reports in Italy suggest they are asking Villarreal for a reduction in that €6m fee.
Villarreal paid Udinese €9m for the defender in July 2011.