Transfer of sponsorship

i am working in a Foreign Engineering Procurement and Construction Company which have an ongoing project here in jubail. I was working as a free visa before until the saudization take place. so in short i change sponsor and join a supply company in a percentage basis. after 10 months i have experience  problems regarding my salary, so i have decided to talked to this supply company and tell them what i will do. i will change again my sponsorship to much qualified company. but my company said that i have a 2 year contract with them. but when i see the contract that they are telling. it was a employment offer and not a contract. My question is can i transfer to other supply company knowing that i does not have a valid contract with them.