According to the "Notice of the Ministry of Finance and the State Administration of Taxation on Several Policy Issues on Business Tax":

If the price and discount amount are stated on the same invoice, the discounted amount will be used. The price is the turnover; if the discount is invoiced separately, it shall not be deducted from the turnover regardless of how it is handled financially.

If the taxpayer's turnover is calculated by deducting the turnover due to a refund after paying the business tax, the business tax paid shall be refunded or deducted from the taxpayer's future business tax payable.