An offer letter has very little meaning or value. A contract attested by the Labour Office does. Since you have not signed a contract, the company may only be able force you to buy your return ticket and nothing more than that. You do not need to give even a month's notice as there is no legal contract between you and the company yet. You could give them say a week's notice. Just make sure you collect your salaries before putting in your resignation letter as the company may hold your salaries back, and you will be left helpless.