Is it permissible to obtain the agreed-upon salary increase secretly, without the company's knowledge or consent, and in a secure manner that prevents discovery?

What the questioner asks about concerning taking his right discreetly (the issue of az-zafar) relates to a promise from his company to increase his salary, and this promise led to the renewal of his contract. If the company does not fulfill its promise, he has the right to terminate the contract without any harm befalling him. And if terminating the contract causes him harm, such as deducting his financial entitlements for his previous service period, then this is an injustice.

The most appropriate matter to be considered for the issue of az-zafar is these financial entitlements, not the promised increase amount, because the company may have an excuse for not fulfilling the salary increase, but it has no excuse for deducting his entitlements if he terminates his contract, which he renewed based on this promise.