Modifying lineItems discount

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I'm willing to update lineItems Discount amount on a BOGO order. This aim to refund / return the correct amount based on the discount on the it's own. and no specific items.
With the current design, a customer can return the 'not-free' item and get an Item for free. We don't want that. 

Here's the use case:

Screenshot 2021-02-18 at 10.36.15.png


as you can see the total discount is not changed.

Do you guys have any way to do that?
I've seen and I'm aware that we can't modify the lineItems.
We've built a whole OMS as a workaround to this limitation, 

But by curiosity is anyone got an idea of a way to do that, expect from building a whole OMS?