That would effectively be a new contract though, and he would likely want a hefty price rise and a nice signing-on fee to compensate for the loss of flexibility.
They can't, that is what a release clause is. If we meet it they have to agree to sell at that price. However it doesn't mean that the player will agree personal terms, or that another club will come sniffing around too. If they didn't want to sell him they shouldn't have put that clause in the...
It doesn't look like Joe Mason will be going anywhere until after the next round of the cup according to Bolton News, also a £600,000 price tag mentioned.
http://www.theboltonnews.co.uk/sport/10936461.FA_Cup_victory_could_spark_arrival_of_Cardiff_City_duo_for_Bolton_Wanderers/