Fear of selling is a big problem as you work your way up the leagues. That cash could go a long way to rebuilding your entire squad for the challenge of playing up a division.
Just remember that if he's really that good you won't be able to keep him forever because either he'll want to leave or your chairman will simply sell him on you
Unless of course you're some sort of uber-manager and your upwards progress matches his ambitions. (Yet to experience this phenomenon

)
(I'm not bitter at all about the chairman bit

)