杜佑
In 806, when Yang Huilin () the nephew of Han Quanyi () the military governor of Xiasui Circuit (夏綏, headquartered in modern Yulin, Shaanxi), seized the circuit after Han was recalled to Chang'an, Emperor Xianzong was discussing with the chancellors how to react. When Zheng submitted suggestions, he invoked an ancient saying that stated that soldiers of Xiasui's capital prefecture Xia Prefecture relied on county officials, which confused the other officials, and they believed that while Zheng was scholarly in ancient matters that he did not have skills to deal with the current emergency. Further, at that time, there was a senior secretary at the legislative bureau, Hua Huan (), who was closely associating with the powerful eunuch Liu Guangqi () such that whatever Hua suggested, due to his seniority and powerful connections, the other chancellors Du You and Zheng Yin would usually not dare to oppose it. Zheng, however, was displeased that Hua was overstepping his authorities and rebuked him. As a result, Zheng was removed from his chancellor position in summer 806 and made an advisor to the crown prince. However, when Hua was found to be corrupt later that year and executed, Emperor Dezong, finding out that Zheng had rebuked Hua previously, was pleased with Zheng and made him "Guozi Jijiu" (), the principal of the imperial university, and soon made him the mayor of Henan Municipality (河南, i.e., the eastern capital Luoyang). In 808, Zheng was made the defender of Luoyang.