If presented with a situation where you need to refund some, but not all items off a finished sale you need to follow the steps below.
First follow the steps to find and refund the original sale
This returns all items to stock and refunds the entire sale amount to the payment method they provided originally
Once that is refunded you need to re-enter the items the customer is keeping into a new order and tender payment.
Lastly if the item they have returned cannot be re-sold and you're tracking stock ensure you go to Menu > Add Ons > Wastage and record the item, reason and quantity that was wasted.