In my company, we people signed the sacking paper...

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Please note that my answer to your question will be purely from an academic point of view based on the limited initial information provided.

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In refference to the Article 28 of the Labor Law in the Private Sector (No. 6/2010) which forbids employers from reducing the salaries of their workers, states that even of the work contract is for a limited or unlimited period,
the worker’s wage (salary) shall not be reduced during the validity of the contract.

Any agreement signed before the validity of the contract or subsequent thereto shall be deemed absolutely void as it is related to the public order.
any other agreement which is contrary to the law or to the public order, workers has the right to annul it through the courts,