Doing personal work out of office time

My question is that "freelance work is allowed in Islam or not?" Let me tell you the details. I have signed the contract with my company and in my contract it is mentioned that I cant work for any other company until unless I have written permission from the CEO of the company. Now I got one freelance project which probably I will do after my office timing at my home so my office work will not be effected with this. But sometime I also do work in my free time in the office using office machines. As in the start of Surah Maidah is " 'O believers! Fulfil your promises" so on that I guess I can't but I have still confusion. In the end, if I am doing this, I will earn with my work and one of my friends told me this kind of contract is similar to slavery and slavery is not allowed in Islam, so you can violate that contract. Kindly help me out in this as soon as possible.