@Inna22 As others have mentioned, I just don't understand asking for a discount because of a mouse sighting! It's just one of those things that happens sometimes and doesn't mean the end of the world. They saw a mouse. They were not inconvenienced in any major way. Or even in any minor way, IMO.
For what it's worth I think you did the right thing by offering champagne (a very hospitable gesture for the "eek" factor), and I think they are being unreasonable by asking for a discount.
But since the can of worms has already been opened, I would not ask them what they think is appropriate and instead, go ahead and offer what YOU are comfortable with. You say it's a high-dollar reservation so I wouldn't offer anything more than amounts to half off ONE SINGLE NIGHT.