A debtor may request the Central Collection Unit to investigate a debt: after notification from the Central Collection Unit under § 13-914 of this subtitle that the Central Collection Unit intends to request the Comptroller to withhold the debt from any income tax refund due the debtor; or within 30 days after notification from the Comptroller under § 13-915 of this subtitle that a debt has been withheld. On receipt of a request for an investigation, the Central Collection Unit shall investigate any questioned debt with the State agency that referred the debt. The Central Collection Unit shall make a written determination within 15 calendar days after it receives a request for investigation from the debtor. If the Central Collection Unit determines that a referral or certification is in error, it shall, as appropriate: correct the referral or certification; discontinue certification procedures; or promptly remit to the debtor any amounts that have been improperly withheld.