No locality receiving state funds under § 53.1-85 shall use such funds for any purpose other than for paying expenses incurred as the result of the confinement of persons in local correctional facilities. The Department shall require a locality to return any portion of state funds expended in violation of this provision to the state treasury. Should an unexpended balance of state funds exist at the end of the apportionment year, the unencumbered funds in such balance may be reverted to the local treasury and subsequently shall be expended for operating expenses of local correctional facilities. In the case of regional correctional facilities, the unexpended balance of state funds shall be apportioned by the regional facility’s governing body to the participating localities based on the number of prisoner days of persons confined in the facility from each jurisdiction. Each locality shall keep records of receipts and disbursements of state funds received pursuant to § 53.1-85. Such records shall be open for evaluation by the Department and audit by the Auditor of Public Accounts.