Yes it's up to them if they want to sell at a lower price. Messi has a £630 million release clause but if you offered Barcelona £100 million they'd snatch your hand off.
It says Orlando requested and Bruge didn’t want to pay. So I’ve understood that as Orlando was willing to sell for that price.
Anything can be negotiated but you would have to question why if someone has a 20 million release clause but the selling club asks for 8.
I suppose it's to ensure they get a good price for him if he does turn out to be good. If someone really wants to sign him, they can for $20m and it won't be refused. It doesn't necessarily mean they'll only sell for that price, but they can't reject an offer above that. If he scores 15 championship goals this season, then that clause is going to look very poor value for them then