How can I easily send a replacement product for free?

Hello, I am also trying to do this manual draft order. However, I want to refund the original product and then give my customer a free product. If I ‘mark as paid’, then yes, the customer has the product for free, but in my accounting records, it shows it as ‘income’, which I collected outside of Shopify.

What I would like to do is to mark is as £0 received, so that I know there has been no actual money received for this order. I can’t see a way to do this (without using a discount code). Is there a solution?

Thank you