Refunding Usage Charges from a Sale Channel?

Shopify Partner
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Hey there. I'm developing a Sales Channel app and will be taking a commission on sales generated through the app via Usage Charges. If a customer requests a refund and the merchant refunds the order, I'd like to also refund the merchant the corresponding Usage Charge. I didn't see any way to do that with the API?

How is this typically handled? If we issue a Credit instead of a refund, will Shopify take their commission on the same $$ when we charge them for the Credit money on a subsequent sale?