Should I refund for one dog two days before the booking begins?

I was booked to housesit with five dogs from a Saturday morning-Wednesday evening. On the Thursday before, the owners told me they may take one of their dogs on the trip with them but they hadn't decided yet. I arrived today and they had taken the one dog with them, so I am only watching four of the dogs when I was booked for five. My cancellation policy for housesitting is "strict," and they didn't request or mention a refund, but this is my first time booking with them and I thought it could be polite to offer. Has anyone else dealt with this?