Answer: One of his kidneys. According to ShanghaiDaily.com, "On April 28 Zheng went to Chenzhou City in neighboring Hunan Province for the kidney removal surgery arranged by the broker. His parents knew nothing about it, Zheng said. He was paid 22,000 yuan after his right kidney was taken out at the Chenzhou No. 198 Hospital."
The article goes on to note that the hospital "was not qualified to perform organ transplant."