Castillo is contracted to Olympiakos until 2010, but they are ready to offer him a double-your-money contract in order to fend off big interest from around Europe.
The 23-year-old, who agreed to join Manchester United as a 16-year-old before a work-permit wrangle ruled out the move, is now the subject of immense interest.
Greek sources claim both Shakhtar Donetsk and City have lodged offers for the player - but Olympiakos are ready to fight hard to keep Castillo.
The midfielder is married to a Greek woman, which Olympiakos hope will mean he is likely to stay - but it also frees him to move across Europe as an EU national without any restrictions.
Castillo is due to meet Olympiakos officials on Tuesday to discuss his future.