S.C. Code Ann. § 12-60-490

If a taxpayer is due a refund, the refund must be applied first against any amount of that same tax that is assessed and is currently due from the taxpayer. The remaining refund, if any, must then be applied against any other state taxes that have been assessed against the taxpayer and that are currently due, or offset as provided in Chapter 56 of this title, or offset to collect a debt pursuant to Section 12-4-580, or both. If any excess remains, the taxpayer must be refunded the amount plus interest as determined in Section 12-54-25, or, at the taxpayer's request, it may be credited to future tax liabilities.