What can I do if my employer wants to change my salary listed in an offer letter?

I work as a contractor in a government agency for Company XYZ. I hold 2 separate positions. My primary position is a Help Desk Analyst (M-F), my secondary position is a NOC Analyst (Sunday 12 hour shift). I repeat, these are 2 separate positions. When I started with Company XYZ, I was given an offer letter which offered me the Help Desk Analyst position for a specific amount. The letter clearly states it is for that position, and only that position as it does not mention my Sunday shift. When I receive my paychecks (yes, plural, as I have been dealing with this for quite some time), I notice that while I am working extra hours with this second position, I am not getting paid extra (and on the other hand, I do not get pay deducted if I don't work on Sunday). When I brought it up to them, they state that what they "meant" in their offer was to include all my hours (ie. 52 hours a week), even though that is not what the offer letter states. The offer letter clearly only states Help Desk Analyst. Now, fast forward to today, I am no longer working the Sunday shift (as I still get the same pay), and they want to lower my Help Desk Analyst salary rate by approximately 20%. What can I do?