(a) All installment agreements authorized under subsection (c) of Code Section 48-16-5 shall bear interest on the outstanding amount of tax due during the installment period at the rate prescribed under Code Section 48-2-40.

(b) Notwithstanding the provisions of this title, if any overpayment of tax under this chapter is refunded or credited within 180 days after the return is filed, no interest shall be allowed.