<human>: Provide a brief overview of the text.

So I am just starting a second, year-long contract with my employer. I transferred cities *six months* ago, (as a foreigner in a non-U.S. country). That is important because I transferred from a city in which my employer adjusted everyone's pay because of new tax contributions in that city (i.e. I would have got paid more). But I am not in that city, and my pay was never adjusted because I moved out before the adjustment would have been made.
Fast forward six months later, I get an offer letter explaining the benefits of signing another year-long contract. In this offer letter I see that the bonus equivalent of one month's pay is more than I was originally getting for one month's pay. I thought, "hey, what a nice raise", until I got paid. The month's salary and bonus was the same as the previous month's salary.
So I emailed the person who signed off on that offer letter and they claim it was a mistake because I moved from that city in which everyone's pay was adjusted for an increase in taxes. I explained to her and her manager that the offer letter states that it takes precedence over all other documents and that I did not make any mistake, so I don't need to pay for this mistake. The big manager replied saying that the person will schedule a time for a discussion with me. How should I approach this discussion? and how should I argue my case?
<bot>: Employer CLAIMs to have screwed up in my favor, now they are trying to walk it back even though the legal documents favor me. Now they want to "have a discussion with me". How should I argue my case?