Partial refunds are not supported by Commerce Interface,
Only the refund of a single quantity of a line item can be processed in CI.
Multiple-Quantity Pre-Fulfilment Adjustment/Cancelations
Fill or kill, no order should be partially fulfilled/shipped
Cancel any order if you can’t fulfill the whole quantity.
Less than full quantity refunds are available in CommerceInterface after an order has been shipped. When marking an order returned in CI you will be prompted to provide the quantity being refunded. Note that the customer will only receive a refund of the shipping fee if the full quantity has been refunded.
For an overview of order management in CommerceInterface, see our Order Fulfillment Guide