Bolton have decided to cash in on Cahill, who is out of contract at the end of the season when he could leave on a free transfer.
The fee for the 26-year-old centre-half is believed to be around ?7 million ($10.8 million).
"There has been a fee agreed between the clubs and we have now given Gary's representatives permission to speak to Chelsea," said Coyle, speaking ahead of Bolton's New Year's Eve home clash with Wolves.
Chelsea are also considering smashing the British transfer record for the second time in a year by moving for Brazil forward Hulk, The Times reported on Friday.
The paper said Chelsea manager Andre Villas-Boas was mulling making a fresh attempt at prising the striker away from his former club Porto, who have placed a 100 million euro (?84 million) buyout clause on the player.
Chelsea have lacked thrust in attack with Fernando Torres, who cost a record ?50 million in a move from Liverpool in January, struggling to find form, and Didier Drogba leaving to play in the Africa Cup of Nations.