Is it permissible for me to take the price of the meal designated for me during my work at the restaurant, or to take the money for myself, without the employer's knowledge, in exchange for not eating it and buying something else with that money?

It is not permissible to take anything from the restaurant's money without the owner's permission, whether as compensation for a meal or for any other reason, because this is an act of aggression and a breach of trust. Allah has commanded the fulfillment of trusts and forbidden treachery, as it is stated in the Quran and Sunnah that "There is no faith for one who has no trustworthiness," and "It is not lawful for a person to take his brother's property without a rightful claim."