Without question it happens.It probably does happen. If a club want to sell and the player refuses as the offers less, I bet the current club will make deals with them to cover the players potential loss of earnings as it will save them money in the long run instead of the player just running down their contract.
The other way would be they accept less transfer fee so that the buying club can afford more in wages etc.
My point is that if United sold Maguire to West Ham for example, they do not have to pay him the remaining wages.
It will be up to HMs agent to get the best deal he can for his client from West Ham.
Now lets say West Ham offer him 160k a week and he wants his 180k, he could go to United and say "Pay me the 20k a week or im gonna sit here and you dont get your transfer fee".