Because your contract was for a year, if a year has passed, the contract time has finished and a letter of release would not be needed. I have heard
stories of people having a hard time upon re entering the country if they "did a runner", however I have also known
people who did it and had no problems. My advice, for what it's worth, would be to check out any new prospective job in detail before you commit to it.
All the best,