Let’s say my bill is $800. 
I pay $400 with credit card and apply the bank feed towards the bill. 
Balance from unpaid bill is $400. 
So far so good. 
Item received wasn’t correct. We return. Vendor issues full refund of $400 on credit card.

My end result should be an open bill for $800 because the credit card charge and refund offset each other. 

This is so simple but it appears to be undoable in QBO. 

If I apply the bank feed of credit as Credit card credit, it will come negative (just like the charge) and the balance on the bill will be $0 ($800-$400 charge-$400 credit)

Vendor credit will not work either because it will also come as negative and make the balance $0 just like credit card credit.