In 736, Niu replaced Xuanzong's second cousin Li Hui () the Prince of Xin'an as jiedushi of Shuofang (朔方, roughly modern Yinchuan, Ningxia), and the official Cui Xiyi () replaced Niu as jiedushi of Hexi. Cui was impressed with the amount of treasury as well as supply of armor and weapons that had been stockpiled in Hexi, and he reported this to Xuanzong. Xuanzong sent the official Zhang Lizhen () to verify this, and once confirmation had been sent, was exceedingly pleased. He wanted to promote Niu to minister of defense (兵部尚書, "Bingbu Shangshu") and wanted to create him a title—both actions were opposed by the chancellor Zhang Jiuling, on the basis that Niu, not learned, was unsuitable to be minister of a major ministry, and that being honest and frugal were part of his responsibility, not something that should be awarded with a title. This drew Xuanzong's displeasure, and another chancellor, Li Linfu, seeing this, advocated for Niu's creation as a duke. Xuanzong agreed and created Niu the Duke of Longxi. Later that year Zhang offended Xuanzong further and Xuanzong removed him and his friend and fellow chancellor Pei Yaoqing from office. He made Niu the minister of public works (工部尚書, "Gongbu Shangshu") and designated him as "Tong Zhongshu Menxia Sanpin" (), making him a chancellor "de facto", to serve alongside Li Linfu, while still letting him keep the post in Shuofang. As chancellor, it was said that Niu was careful and frugal, and did not dare to make any key decisions, deferring all of them to Li Linfu.