Can employer lie about one's salary, relative to others? Are there remedies?

I was hired for a position, in which I have always thrived, as reflected in the positive words of colleagues, managers, the executive director, and performance reviews (nothing short of a 4/5 by any measure, and 5/5 in the overwhelming majority). Some time into my employer new employees came in with the same title, but I became de facto manager. My boss talked about at some point making it formal, but that's not my concern. I objectively have a higher workload than anyone in the office, including managers. This is clear through a system we have that tracks assignments. During salary negotiations I cited my workload among other things and requested a raise. I got a raise, but the number offered was declined, because "you make more than the others and if I raise your pay, I have to raise theirs." I believed her and even though I never would have mentioned others' salaries, her doing so made me accept the offer. I learned I am the lowest paid employee, performance notwithstanding.Now what