The gross proceeds of sales of tangible personal property delivered after the imposition date of the tax levied under § 4-10-20 in a county, either under the terms of a construction contract executed before the imposition date, or a written bid submitted before the imposition date, culminating in a construction contract entered into before or after the imposition date, are exempt from the local sales and use tax provided in § 4-10-20 if a verified copy of the contract is filed with the South Carolina Department of Revenue within six months after the imposition of the local sales and use tax.