Just on the contract part.
In pro football contracts, they basically boil down to:
- on the club side, once the player signs on the dotted line, he gives up his time for the duration of the contract to the club, that includes training sessions, football matches, travel, etc..., Regardless of his status, whether he is a regular starter, rotation, bench player or sent to the u21.
- on the player side, he is entitled to every penny in the contract as long as the contract is valid, no such thing as eerned not earned, once the club and player sign that contract, the player is entitled to the entirety of the money stipulated in that contract
From there, everything is then negotiable, if the club wants a player to leave, he is entitled to ask for every penny of the current contract, if the player wants to leave, the club is entitled to ask for any fee, so to facilitate an exit, both sides (club and player) must negotiate, sometimes a player would be getting a bigger contract in the next club so normally he wouldn't ask for the reminder of his contract to be paid upfront because the selling club can say no and he would risk losing the bigger contract, on the flip side, a club may offer an upfront of reminder of the contract (or part of it) to incentivise the player to move on to another club that is offering less money, and sometimes a player who could be moving to lesser team and will surely get lesser money might ask his current club to let him go for free or small fee, there are also other possibilities, but purely on principles of agreement in the contract, both sides have entitlements and a negotiation is needed to reach a compromise.
Now back to Maguire, not sure what's true but on principle, he is entitled to demand the money that remains in his current contract (or at least the difference) if he is going to a club that will pay him less than what he currently earns.