If you are allowing the guest to take their time to make a decision about the special offer, maybe you should manually block those days and reopen them if/when the guest decides to book with you.
Thanks, the problem as i saw it was, I did have the day blocked until she said she could booik, then I unblocked and sent her the offer, but there is nothing that would prevent someone else from booking that day before she had a chance to accept the offer. There should be a way to send a special offer and prevent others from booking it.
As long as you keep making use of the Instant Booking option, the priority will go to the guest that pays the first. The responsability falls in the guest interested in booking the place. I dont think many hosts would agree in holding days for a guest, even though there are some exceptions.
At this point, it will be a matter of you two coordinating the opening dates/making the reservation process.