Ok ... I think I understand where you are getting confused ... line 18 should show the $5000 that you applied to the 2020 return ... that is a done deal, it cannot be changed ... it is just like if you had gotten the refund and then sent a check to the IRS.
Then the balance due you owe is on line 20 ... this is the amount you need to send with the 1040X. Leave lines 21-23 blank.
