Sportitalia says today Palace tabled a €20m cash bid for the Belgium international.
Atalanta are prepared to sell, but want Palace to raise their offer by €5m - and are willing to take the extra cash in bonuses.
For his part, Castagne isn't rushing to leave Atalanta, knowing it'll mean sacrificing a Champions League campaign.
However, the fullback also has ambitions to play in the Premier League and knows this could be his best chance to get to England.
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