Your total tax liability was not changed, just the amount withheld was changed. On your original return, your tax liability was likely $911. You entered that you had $843 withheld so that left you with a balance due of $68 which you paid with your original return.
When you changed the amount withheld, you still had a tax liability of $911. Except, the actual number was $743 that you had withheld. So if you would have entered $743 on your return originally you would have owed $168.
$911-$743= $168
You paid $68 with your original return, so you are still short $100. That is why it is showing that you owe $100.
You would not enter anything as overpayment on your original return because you did not have an overpayment, you had an underpayment. If you would have had an overpayment, you would have paid more than you should have instead of less than you should have.
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