Gee, doesn't sound like a sector duke to me. This sounds like the ruler of a powerful Imperial member world, though:from Star Wars, but sounds perfectly like a sector duke to a local world's head of state...
"That's not fair; we had a deal!"
"I have altered the deal... pray I don't alter it further."
"My Lord, this sector is not your property by divine right. It is a trust given to you by the Emperor. Your noble title comes at a price. It obligates you to treat your charges fairly and according to the laws and customs that govern interaction between the Imperium and its member worlds. I shall be bringing charges of malfeasance against you in the Sector Court, and I shall also make sure the Emperor is apprised of your breach of faith."
As I point out every time you bring up that old "The Imperium is a government of man, not of laws" catchphrase, the fact that the Imperium have courts and judges show beyond any dispute that the Imperium does have laws. So does the handful of Imperial laws and regulations quoted in various canonical sources. Chris Thrash has a whole list of them on his website. Go take a look if you don't believe me.
Hans