It all depends how far it got. If they only signed a Heads of Agreement then there's not much to worry about. Here's an explanation of what a Heads of Agreement is:
"A Heads of Agreement is a good document to start with when you are planning to or are interested in entering into an agreement with another entity in the near future. Although non-legally binding, it is useful for keeping a record of what was discussed during the negotiation state, and what is to be included in the proposed agreement. If the proposed agreement has a signed paper record, you are able to ensure that the execution of a legally enforceable document is smooth and minimises any potential conflict."
https://lawpath.com.au/legal-documents/heads-of-agreement
All Hasler could do is to argue that he was given a verbal and/or written agreement which prevented him from getting another contract with another club but he'd have to provide evidence that he was given an actual offer by another club. He still wouldn't be able to get $2 million though. At best we'd have to pay him for time lost which would be a maximum of 1 year pay on the offer he received but no more than the final year of his previous contract.
Basically put, there's a chance we may have to pay him $1 million but in all likelihood we won't pay him a cent.
There are some exceptions though. A Heads of Agreement can be binding if the wording wasn't done properly. For example; if the Heads of Agreement states that the contract will automatically accepted if not finalised at the end of the agreement period then it becomes legally binding. This also works if there is a guarantee in there that states something like, "this agreement will be honoured unless terminated by both parties" (otherwise known as due diligence)
But honestly there is very, very little chance that a legal team would be stupid enough to put that in there. Those mistakes you see in small business sales when a sole proprietor wants to sell the business and just signs the Heads of Agreement without legal representation. Not something a major club would do.