I appreciate you for sharing detailed information about your concern, Erss.
Allow me to make some clarification on recording refunds in QuickBooks Desktop.
In QuickBooks Desktop, we don't support by kit. We can only adjust if you record the inventory as individual parts. You may also return the entire item, but you'll have to adjust its value since you were only charged for half.
However, I will suggest consulting your accountant. That way, they can help you determine what's the best way of recording this type of transaction.
If you don't have an accountant, you can check our ProAdvisor page, and we’ll help you find one from there.
In addition, I've added this article to learn more about payback or handling your customer's credits in QuickBooks: How to create a credit memo or refund check in QBDT for Windows.
Keep in touch if you need any more assistance with this, or if there's something else I can do for you. I've got your back. Have a good day.