Did my manager take advantage of me?

So I’m a server at a restaurant. Normally on Saturdays and I make $300 per shift. Today was an event and they put me on a task where my cap off would be $120 at most because it was hourly pay. They did not tell me ahead of time that I would be put to this task. They wouldn’t take no as an answer as well when I wouldn’t take the pay cut for the sake of them needing someone to do this task even though I wasn’t told a head of time that I would be working a job that involves a pay cut. Did they try to take advantage?
Did my manager take advantage of me?
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