Cahill was the subject of much interest during the summer transfer window, with Chelsea, Tottenham and Arsenal all linked with a move for the centre-half.
However, Bolton's £17m asking-price for a player in the final year of his contract appeared to scare off potential suitors.
As interest mounts around Europe, with Barcelona and both Milan clubs reported to be interested, the Wanderers are expected to sell the centre-back at a reduced price this winter.
However, according to The Mirror, Madrid are interested in securing the England international's services for free.
Should he choose to move abroad, Cahill would be able to sign a pre-contract agreement at the Bernabeu and complete a Bosman transfer in the summer.