During the process of the credit sales, the enterprises always use cash discounts to give the buyer a certain concession according to the length of payment periods, in order to encourage the buyer to make a payment as early as possible and reduce the risk of credit sales. However, in the course of the credit sales, it is obvious that the buyer's credit rating is the major factor in managing the credit risk. Therefore, based on the Incentive Compatibility Mechanism Design Theory, this paper not only establishes the corresponding sales discounts according to the buyer's credit, but also performs a pricing analysis on them. In addition, under the premise of making basic assumptions for both sellers and buyers, it establishes the profit function and the constraints of the credit sale enterprises, and uses the optimal game theory to works out the buyer enterprises' sales discount function, for providing a theoretical basis of making a reasonable credit sales decision.