Is it permissible to terminate an employment contract binding for two years after one year has passed, given that the financial increase received would be returned, knowing that terminating the contract is considered a breach of a promise made to the employer?

The contract of ijarah (definition) (leasing/hiring) is a binding contract according to the majority of scholars, and neither party has the right to unilaterally terminate it except with the consent of the other party, unless there is an unforeseen circumstance that prevents the utilization of the benefit. However, if it is merely a promise to work, then it is obligatory to fulfill it, especially if its non-fulfillment would cause harm to the other party.