Partial refund



I was wondering if anyone has an easy solution for my problem.

I rent out an apartment in Reykjavik and my neighbor was renovating his apartment last week. Changed the windows and something like that. Anyway they have not taken down the scaffolding and the yard is off limits because of construction debris that will not be taken care of until next week because one of the windows came broken so they need to replace it again. 

So I have offered my guests who arrive today a 20% discount due to these circumstances. Is there any way for me to make that partial refund through booking.com?