Is it permissible to take from the restaurant at the financial discount rate that occurs due to the "cost accounting" clause, in the absence of a Sharia violation or theft, or must one leave the job due to backbiting, slander, and bullying?

You are not allowed to take anything from the store without the owner's permission under the pretext of a deduction, because the permissibility of taking a right without the owner's knowledge is conditioned on that right being established, and its owner being unable to obtain it by any other means. The deduction from the worker's salary due to low sales is an invalid contract, because it leads to an unknown wage for the worker. Therefore, the worker is entitled to a fair market wage (ujrat al-mithl) estimated by experts. If the employer refuses to change the contract, then leave him and seek a lawful livelihood.