How does my customer get a refund if they cancel partway through?

A family left their lovely dog with our family for three weeks to go to India. He has been such fun. Anyway, that it not relevant. Sadly, their daughter became very ill and they had to cut their trip short and told me that they will get the dog about a week earlier. Is there a way in the app to cancel the booking so I keep the money for the time he did stay with us, and they get refunded for the time they paid but now don't need sitting? In the end, I could just write them a personal check for the balance, but I like to do everything through the app (and there is no reason why Rover should get the money if I am not working for it!)