All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
First of all, we ask Allaah to reward you for seeking lawful means of sustenance and fearing to commit what is forbidden; indeed you did well. A soul will not die until it consumes its provision and completes its appointed time. Abu Umaamah narrated that the Prophet said, “The Holy Spirit (Jibreel [Gabriel]) has inspired to me that no soul will die until it has completed its appointed term and received its provision in full; so fear Allaah and be moderate in seeking sustenance, and no one of you should be tempted to seek sustenance by means of committing sin if it is slow in coming to him, for that which is with Allaah can only be attained by obeying Him.” [Abu Nu’aym - Al-Albaani graded it saheeh (sound)]
Al-Munaawi said in Faydh Al-Qadeer (while explaining the hadeeth), “(Fear Allaah) means that you should trust His guarantee, but He ordered us to seek it from His Lawful provision; it is for this reason that He said, 'and be moderate in seeking sustenance;' i.e. seek it with good and lawful ways without exhausting yourself on getting it in an unlawful or suspicious manner.”
As regards what you mentioned about your employer failing to fulfill their promise to increase your salary although you carry out voluntary work that is not among the requirements of your work duties, then this does not allow you to take anything from your work without their authorization as this would constitute a betrayal and is not permissible because this is consuming their wealth unjustly. Allaah says (what means): {O you who have believed, do not consume one another's wealth unjustly.} [Quran 4:29]
So do not do that, and when the duration of the contract between you and the company ends, it is your right to refuse to renew the contract unless they increase your salary. Also, you may refuse to do additional work that is not part of your work duties agreed upon in the contract or according to customary practice unless they pay you a fee in return for doing it.
If you do this additional work voluntarily without asking for a fee, then you are not permitted to take anything without their knowledge, even if there is a profit for your employer in what you do.
Allaah knows best.