It's a bit tenuous but the manager's deal will usually contain clauses saying that he won't speak badly of the club or its senior management. After his comments yesterday, United might want to argue that he breached the contract in order to bargain down on his pay-off (although I'm not convinced they'd win if it went to tribunal).
Their official statement says that he has "departed his role" and that "the club’s leadership has reluctantly made the decision that it is the right time to make a change". This doesn't say that his employment has been terminated, and they may have just put him on gardening leave. Or it may just be that they're just taking a few days to sort it all out.
But gardening leave is quite a common tactic as it means if someone comes along and wants him to take over their club then they have to do a deal with United to release him (which potentially saves them on the pay-off).
If they do have to pay him off, then they'll need to write off whatever is left of the £11m fee they paid to recruit him (rough estimate is £6.4m left, of which £2.1m would be charged this year anyway, so an extra £4.3m), plus the pay-off for him (and probably his staff too) which is going to hit their results pretty badly.
As for replacing him, and I think we see this with Chelsea appointing Rosenior, there doesn't seem to be much managerial talent available right now. That may well be another factor in Slot keeping his job.
