Can an employer reduce wages for work already done?

Question Details: I recently quit my job. When I started working the owner told me that I would be paid $13/hr; I have this in a text message from my employer although there is no contract. However, after I let them know I would be quitting due to unsafe/unclean working conditions, they told me that I would only be receiving the minimum wage $9.89 for time that I had already worked. They also told me not to come in for my final shift. This was after I had worked my next to last shift. According to my boss, my work was terrible and so they would not pay me the promised amount. Is this legal?

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