post https://sandbox.asaas.com/api/v3/installments//refund
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It is possible to refund an installment received or confirmed via credit card.
Just like the process of refunding a standalone credit card billing, the corresponding balance of the installment is debited from your account on Asaas, and the billing is canceled on your customer's card. The cancellation may take up to 10 business days to appear on your customer's statement.