To manage this type of refund there is a panel Walmart Refund:-
Select Walmart > Walmart Orders > Refund from the menu section to open the Refund Management page.
Click on Add New Refund button to open Refund Information form.
Then enter Merchant Order Id or Reference Order Id to refund.
This will open a form for entering the Refund related information for that particular order.
Fill the refund details and submit.
A Success Message will be displayed after some time.