The calculations are all correct.
The $100 appears to be the donations made to the various causes from form I-330.
Your SC tax on line 10 comes from the SC tax table and matches what is on the tax table
Your solar credit carry forward as per SC rules is 1/2 of your tax and is showing on line 13 of your SC-1040
Your withholdings are $320 less than your tax liability which is where the $320 comes from
Then on line 28, $100 is listed for total contributions from I-330, this increased your amount due to $420.
It appears as though the state is refunding your $100 for contributions.
You would need to contact them to see why.
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