Hi Lucienne, For a respectful and likeable guest, I would refund the difference. You could, however, compromise by letting him know that you will refund any days you are able to book the room. Because you post a discount on weekly stays, won't you get a higher daily fee for those days? You can also increase your chances of booking them by giving a discount--we do this because we'd rather have some of the income than none of it. Hope this contributes to your decision process.
Hosting since Thanksgiving Day 2012 and over 1,000 stays. May you ever be the benevolent ruler of your own domain--YOUR life. Allow no other to rule over it.