Player power? It's general employee decency. In most jobs, if you had the offer of a better position on better pay, your current employees wouldn't/couldn't block you from taking it.
Regardless of any claims of "overpaid prima donnas" etc. etc. and any other criticisms footballers get, they are still just people who want to excel in their particular line of work and they should be allowed to move on to another more illustrious position with better pay if they want to. I would see nothing wrong with Adam forcing his employers to let him have a chance at a bigger club, especially given how short a footballer's career is and this could be the only chance he gets.
I have no idea what your line of work is, if any, but if you were offered a pay rise and a better position than your current job would you turn it down saying that you "work at this job and should honour your current contract"?