Hmmm. if the compensation is greater than your transfer budget, what Fids said should hold. The board won't, in my experience, allow those transfers to go through.
You'll have to keep a closer eye on the info at the bottom of the transfer screen. It does clearly state at time of transfer how much compensation is due.
Ethically, deliberately tying up the player, cancelling at the last minute and signing when they get released is on shaky ground
IMO. Either get pre-contracts and go through with them or wait until the player is released. The one exception, for me, is if the board inexplicably reduces your transfer budget between agreement and completion dates, meaning they cancel the transfer for lack of funds. (If you can see it coming though...)