After joining the company - my designation/title was downgraded

I have been hired by a Fortune 500 Company for a senior manager position via hiring agency. After 9 months - I was told that they hired me at a higher salary grade and i need to work at the lower designation/role(which in fact i was doing for the last 9 months already without any reasons). When i discussed this with my supervisor that why it was not told to me explicitly - they told me they never had that higher role. But my offer letter and the company organization chart depicts me at the higher(original) designation for which i have been hired for. So what are my options - 1)Quit 2)Get justice for myself?After i discussed with my manager-the manager started disliking me and started talking about my behavior. Hiring agency has washed their hands as they had already made their money. Please advice. thanks
It's not only lower designation/title but overall roles and responsibilities are also lowered. It's almost going back in my career some 6 yrs back. If i were aware of this fact-I won't have left my previous job and joined this company. Ethically they should have honored the role/designation they had mentioned in the offer letter. Yes, they are paying me the same salary and benefits they offered me because they don't want to get into the trap of Department of Labor.
People of other races and nationalities get what they have been told and only people of Asian nationalities are being asked to compromise.