Can an employer change your offer letter if they consider it a contract after employment?

I got an offer letter from the company that I'm with. The recruiter told me that I would get a supervisor position, but I did not. On the offer letter I was told that I had the next position down. But I excepted it due to the pay I was offered. I've been working for the company for approximatley 8 weeks and now they are saying that they gave me my position but accidently gave me supervisor pay. They want me to sign a new contract stating a lower pay. They have also said that our offer letter stands as a contract for the company which is odd seeing as I am a civilian contractor in Kuwait.