Asking about my Tuition Fee

IMy university made some mistakes on my tuition. They should charge me as out-of-state student for this semester, however, they billed me as in-state. Last week, they found our this problem and asked me to pay the full out-of-state tuition. I said no and argued with them. But they said it is the LAW, they can't break the law.
What should I do ?

I really appreciate your kind help.

Thank you!