Should I offer a partial refund if my guest modifies his reservation?

[Becca1]

Hi all, 

I am a new host on AirBnB with a flexible cancellation policy. My current guest is staying at my apartment but recently told me he will not be staying the last day, and modified his reservation (and I accepted the modification).  

AirBnB has already issued a payment for the full/original stay. So, a few questions: 

1. Will AirBnB charge me for the last/cancelled day, in order to refund the cost of the day to my guest? And 

2. if not, should I reimburse him...? Not sure what the custom is for this. Although I assume I would do it through the resolution center here: https://www.airbnb.com/help/article/165/how-do-i-refund-my-guest?topic=237 '

Thank you!