OK, player contracts 101.
Player signs for a club, 5-year deal, £100k a week, £20k image rights. The £100k goes through payroll (tax and NIC up-front), the £20k is paid gross to his image rights company which deals with any tax obligations on the income.
On image rights, the primary motivation is saving tax and keeping costs down, but there are commercial aspects too. The standard player contract contains a basic image rights clause but it's quite restrictive on what players have to do. If there's a separate contract, they can be asked to make more appearances, including appearances on their own (the standard contract clause requires at least three players at any one gig). So the Nivea adverts are a good example - Bobby and Milner did adverts on their own, which would tell you they had image rights deals, otherwise they'd have had 2 mates in the ad as well. Nivea will have wanted just the one player, so it's more attractive to them that we can offer that. The deals also allow clubs to prevent players from doing personal appearances for competitors of our sponsors (the standard contract only prohibits this for the kit deal and shirt sponsors).
Provided the player fulfils his duties under the contract, the club is obliged to pay him the amounts due. If he doesn't, they can argue breach of contract and withhold payment and / or terminate the contract. So Isak, strictly, is in breach of his contract at present but the ability to terminate is on Newcastle's side, and they're not going to do that because they'd lose the fee. Chances are they're not paying him though (or will have imposed a fine = same difference).
In reverse, if Newcastle were to tell him to stay away from the training ground for a sustained period, he might be able to argue constructive dismissal and seek termination of his contract. I've not seen that in practice so it is, I think, a theoretical risk. But it's also why they'll be talking about possible re-integration further down the line - even if they know it's not going to happen, they have to be seen to be open to it to prevent the potential breach of contract on their side. The same would theoretically apply if they sent him to train with the kids for a sustained period. You'd need an employment lawyer to give you a full low-down on the level of risk there, but I know we had a concern about it with the odd player down the years.
Player gets injured, club still has to pay. He may not be fulfilling his player duties but it's because he can't, not because he won't. Player gets career-ending injury, club still obliged to pay but in practice the parties will agree a termination of the contract as it's in both their interests not to drag things out. In that situation, the player will get most of what's due to him with a discount for getting it up-front and also being free to go on and take another job / exploit his image rights however he sees fit.
Parties agree to release the player mid-way through his contract, there will be some degree of compensation paid for loss of future earnings. It's rare to see this because of the values involved, but it does happen with youth players where the sums aren't huge, and usually it's because the kid is a knob who's not going to make it and no-one wants him around anymore. These kinds of terminations are common with managers / coaches.
On transfers:
1. A club can't just sell a player. He has to consent to go. If he refuses to sign a deal with the new club then the existing club is bound by the terms of the contract.
2. In practice, once a player has agreed to sign for another club, his existing contract will come to an end by mutual consent, releasing the current club from future payment obligations (other than signing-on fees - the player is entitled to the money on day one, it's just that the payments are spread). The image rights contract will also be terminated and fees paid up to the transfer date.
3. If the new club is paying him more, that's usually the end of things from the selling club's perspective - there is no financial loss to the player so he has no basis to make a claim for compensation. There may be a settlement if he was due a bonus shortly after leaving and the club may agree to honour that. However, so-called "loyalty" bonuses are usually structured so that they fall due after a transfer window closes, so the payment is effectively for staying with the club, and you don't get it if you do one.
4. If the new club is paying less, there's usually a negotiation for the existing club to top-up the wages for the remainder of their contract. So if his contract has 2 years to go, and he signs a 5-year deal at the new club, he'll only get the top-up for 2 years. This is a negotiation and it doesn't follow that the player will be paid the difference in full. The payments to be made will also take into account the loss of image rights income as well as salary. He may agree to take less for the sake of his career, or because he is moving to another country where the tax situation is more favourable and the net pay is equivalent, or where his bonus arrangements at the new club are more favourable. In any event, the press will say he's taken a pay-cut to move to the new club. That is bollocks.
But the point is that when a player leaves for less money, there is a negotiation and the parties come to an agreement. His playing contract ends when he leaves the club and any further payments are regulated by whatever agreement is put in place when he leaves, not by his original contract as that is terminated by mutual consent as part of the transfer process. There will then be a side-letter covering any future payments - amounts due, timing of payment and usually how they'll be treated for tax. On that latter point, if he goes to another UK club, they just go through payroll as normal. If he moves abroad, they're usually paid gross to his bank account and he has to deal with any foreign tax implications (there's no UK tax as the payment relates to duties carried out abroad, so it's not within the scope of UK tax).
That's the basics. There are complications beyond the above around transfers. For example, we had one deal where the player wanted an up-front payment but the only way we could do it was via a signing-on fee which would have been due every year for 5 years. We instead agreed a higher transfer fee and the selling club paid him an exit bonus, as they had greater flexibility around what they could do. On that point, it does seem generally that foreign FAs are less restrictive around the rules they impose than the English FA so there is often some scope to make things work either side of a transfer where it wouldn't be possible for a deal between 2 English clubs.