@Amir722 "Why would I accept a change like this that would actually reduce my net payout?" I don't quite understand. This guest did stay for one week. So had they booked the entire week to start with, your payout would be the same as what you are looking at now.
You don't have to follow Airbnb's recommended pricing tips, you know. Most hosts ignore them. A weekly discount of 10% seems perfectly adequate to me. I don't offer discounts at all, as my listing is already quite a good deal, and most of my guests book for a week or more anyway. The only reason to offer discounts is if you are trying to attract longer bookings.
I certainly wouldn't piss off this guest by making anything of this with him. That could very well lead him to leave a bad review. I would just look at it as him having booked a week to start with. It isn't his fault you have a weekly discount.