post https://api.mollie.com/v2/orders//refunds
When using the Orders API, refunds should be made for a specific order.
If you want to refund arbitrary amounts, however, you can also use the Create payment refund endpoint by creating a refund on the payment itself.
If an order line is still in the authorized state, it cannot be refunded. You should cancel it instead. Order lines that are paid, shipping or completed can be refunded.