New employees should sign a contract for minimum 2 years of service. In between they want to leave the company, they should pay the employeer a fixed amount decided in the contract. (Recruitment, ticket, immigration etc. expenses). The amount payable should be mentioned in the contract.

After 3 years the emplyoee can join any other company if he wants and the same procedure will apply in the other company. Minimum 2 years of service.

There is no need for a release letter after 2 years.