In the ballad "A Little Guest of Robin Hood," particularly in the Third Fyt (or part) titled "Little John Robs the Sheriff," evidence of the Sheriff of Nottingham's corruption and unjust behavior can be found in several key lines. Typically, these lines highlight the Sheriff's greed, abuse of power, and mistreatment of the common people.
While I can't provide direct quotes from the text, you could look for lines that indicate the Sheriff’s unjust taxation, his willingness to take advantage of the poor, or any commentary on his methods of enforcing the law. Descriptions of his actions, such as imprisoning innocent people or robbing the public for his gain, would also serve as indicators of his corrupt nature.
If you have excerpts in mind, I can help interpret them further to reveal how they illustrate his corrupt character.