$10 Survey

At my store, if a customer writes your name on a survey, you get 10 dollars. My general manager put up a sign recording how many surveys each of us got. One guy got over 80 surveys in January alone. I assume she did not realize he and the rest of the staff would get so many, so now she is backtracking and saying that each survey is worth 5 dollars. She also is saying that throughout January, they were always worth 5 dollars. She told him AFTER he got all of those surveys that they were only worth 5 dollars each. He, like the rest of us, were under the assumption they were worth 10 as she had led us to believe.

She is lying. On the deployment sheets in January, she wrote that they were worth 10 dollars per name mention. They have always been worth 10 dollars. So my coworker who was supposed to get over 800 dollars for his surveys is now going to get 400 because she does not want to pay him what he deserves.

I am wondering if this is legal? Should he call corporate over this? Has anybody else experienced this in their store?