The retail sale of a modular building, as defined by § 58.1-602, by a modular building manufacturer or modular building retailer, as defined by § 58.1-602, shall be subject to the tax authorized by this chapter upon sixty percent of the retail sales price. If the modular building manufacturer has paid such tax on the cost price of materials incorporated in a modular building that has been constructed for sale without installation, it may credit against the tax shown to be due on the return the amount of sales or use tax paid on the cost of materials used in fabricating such a modular building.