Through API & documentation, I do not see a possibility to revert the usage charges made before. We charge merchant a percentage cut from the paid order, but if the order is cancelled, we need to revert that back. I tried with minus numbered price (-10 for reverting 10 USD) but it returns error saying "price should be zero or more"(understandably). Searched through the docs but do not find this case mentioned anywhere.
Checkout our application credit, that's the mechanism to use to refund charges from the app. Do keep in mind that you can only refund an amount that has been actually charged to the store, which can take time (based on their Shopify billing cycle).