In conclusion, if I'm understanding you correctly, Ln 23 of the 1040X indicates the additional amount (if any) that should be applied (credited) to my account at the time of filing of the 1040X return. The amount shown on Ln 77 of the original 1040 return has already been applied and will not change. In my case, since the tax amount remains the same, Ln 23 of the 1040X should, in fact, be zero.
