It's just a ridiculous amount, if his annual salary was really just above 100,000 euros, a million euro buyout for one year remaining of his contract is of course way too much. That won't hold up in any court, unless it explicity says so in the contract.
It's an opening gambit in a negotiation. But a contract must have a value to both sides, and no side can be obliged to terminate it (unless they are in breach, or some clause is triggered), but can be induced, financially or otherwise, to do so. And they are entitled to decide what level of financial recompense is sufficient for them to consent lose that asset. Any court will say that a party to a contract has the right to see the contract fulfilled unless they agree to the contrary (except in the case of force majure, which certainly does not apply here).
Visma aren't as rich as people always assume they are, but even if they were they'd be crazy to pay one million just because Bora feel like blackmailing their rider.
If Visma want to start acting like a football team, trying to lure away athletes while they are under contract, they need to either gain the finances to do so, or limit their ambition. That team's financial limitations were made very clear a couple of months ago, but that is a reality they must acknowledge and limit their aggressive behaviour in the face of.
Blackmail? Where does that enter into it? Uijtdebroeks freely entered into a contract that he is being expected to abide by. If he does not wish to do so, he needs to negotiate a way out of it.
Interestingly, Wielerflits also says that Bora didn't pay the buyout money for Roglic either, Roglic arranged that himself (maybe with the help of Red Bull, I don't know). That makes their current stance even more ridiculous.
It is no business of Visma's where the money came from: they expected to be compensated for the loss of an asset (Roglic's services), and they were, at a level that they chose to accept. Now they must do the same if they want an asset that Bora own. If Bora did good business in getting someone else to subsidise them in that deal, that is good business. It might make Visma jealous, but it has no meaningful effect on the matter at hand.
I don't like to see cycling doing down the soccer transfer path,although I suspect that the recent move to longer contracts, especially for younger riders, is essentially inviting such a thing. But I'll raise that in the transfers thread, rather than push this one off topic.