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It's actually not a law, per se, but contract law. That is, there is an implicit contract in an employment relationship. An employer agrees to pay X to an employee, who will do work Y in exchange for X. Time already worked is worked under that prior agreement, and if the employer paid less than was agreed upon for that time (a retroactive wage reduction) that would be a breach of contract. Again, it's not a specific statute, but rather contractual in nature.
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