Hi, I received the same offer and it works for me twice. I don't remember what button I pressed, but I just accepted the offer and the 10% was applied over the price that I already had. I have read that they send this offer to those are looking for booking in those days of your offer, so I think it improves the visibility of your listing.
In the calendar the offer appear like a gray line, similar when you receive a booking, but instead of green is grey. You can always eliminated or changed in the moment that you want.
I reccomend to take it if they send it, you can earn more than lose.