To my knowledge, an "offer letter" has not much meaning in the legal sense. It is only the Contract usually written in both Arabic and English and which has a the seal of the Labor Office that is recognized and becomes binding on both parties. Since you are not on their work visa, the Contract you have signed has no value or worth and will not recognized by the Labor Office or the court. In the current situation, I believe you can resign from your job and leave this company with no fear whatsoever. Make sure you resign after you have collected your salary as the company may just hold back your payment.