You can always negotiate under the release clause, indeed that's what happen 95% of the time.
What can't possibly happen is a buying club paying more than that sum.
Just to give you an example, a number of star players here - Mbappe, Bellingham, Jamal, maybe more - have €1 billion release clauses. If it became known that one of those wanted out, potential buyers would start by offering loads less but Madrid, Barça whoever could try holding out. As everyone knows, nobody will be forking out a billion (what's more in one go), that sum is simply insurance given the astronomical wages these people are on.