Cahill made his international debut for England as a substitute in Friday's 4-0 victory over Bulgaria, and although he was surprisingly omitted from the team to face Switzerland, his release clause has risen to £17.5 million following his first appearance for his country.
Even at that price, Coyle expects there will be suitors for Cahill, who joined Wanderers from Aston Villa in January 2008 and has been linked with a move to Arsenal in the past. If a big club does make a suitable offer, Coyle will not stand in the defender's way.
"There will come a day, and I hope it doesn't come in January, that Gary Cahill will probably outgrow the football club, given the finance involved in it," Coyle told the Bolton News.
"I firmly believe that he could play for any of the top four clubs. And if that day comes, then Gary would go with our blessing because he's been a terrific servant for the football club.
"The reason those clubs are in for him is because he's been playing consistently well for me as a manager. I have always said I have no qualms with anyone, as long as they pour their heart and soul into their job. And if that's the case, then he would go with my blessing."
Coyle added regarding Cahill's England cap: "First and foremost it's a tremendous accolade for Gary but then on the back of that, for Bolton Wanderers Football Club. I have always banged the drum for him and some people might think I'm biased because I'm his manager.
"But there is no doubt about his quality and consistency and I firmly believe that if you get in the team at international level, it becomes difficult to dislodge you.
"When he got on against Bulgaria the other night, I think he wanted to do everything but after five minutes or so he settled down and I think you saw his qualities. I think that's what he has got to do, just play his natural game.
"He's different class when he does the simple things well and I think he'll go on to have a fantastic international career."