The Australian international agreed a four-year deal with the Premiership side yesterday after the two clubs settled on a price. However, Cahill seems to be unhappy with the terms on offer.
This is the second time in as many weeks that a transfer involving the player has stalled after the player's proposed move to Crystal Palace collapsed a fortnight ago.
Chairman Theo Paphitis has already indicated that any improvements to Millwall's squad for the forthcoming season are dependent on Cahill leaving The Den at a price and he will not be pleased to hear that a second deal is on the verge of collapsing.