Can employer reduce my hourly rate of pay because they say they made a mistake and they are paying me too much?

I would like your opinion please on getting legal representation. Friday at work HR called me in & told me that they made a mistake on my pay rate when I went from floor nurse to case management and they need to reduce it from the $3.00 down to $0.20 raise an hour. I have a email stating the new pay back in July when I accepted the position. I have 1 paycheck at the promised rate of pay. HR solution is for me to take the $0.20 raise ( instead of the original $3.00 hr raise), or for me to go back to my old job as a floor nurse ( probably at old pay rate). My question is should a seek legal counsel and fight for the pay raise I took with the job? Eric didn't know if they could fire me for fighting against pay rate reduction. I asked HR to compromise to make their mistake right by giving me the pay difference of $1.50 an hour, they said they would get back to me Tuesday. I didn't find out until late Friday afternoon, & it was too late to call a attorney for consult.