Has your employer revoked your offer letter?

Pursuant to your question, it may be advised that you may have a valid claim against the entity that offered you employment and later revoked the same. In this context it may be noted that an offer of employment may be treated as a contract if both the employer and the employee have duly signed the same. However, the employment may actually commence if the prospective employee does not face any impediment so as to the issuance of an employment visa/ work permit in the UAE, or if there are no sanction which would normally restrict the employer from employing the employee in the UAE.
In view of the foregoing, you may check if you and the entity as a prospective employer signed your offer letter. Also you may check if the employment contract provides that the commencement of employment is conditional upon there being no impediments on issuance of an employment visa for you. Subsequently, you may also check if the entity had mentioned of any impediment or sanction on you due to which the entity is unable to provide you an employment in the UAE.
Pursuant to the above, you may accordingly consult a legal practitioner in the UAE, if there is an element of breach of contract on the part of the entity and also on the possibility of filing a claim against the entity at a court of competent jurisdiction.

Salary limit to avoid labour ban
I have been working as a draughtsman at an entity in the UAE since March 2014 and drawing a monthly salary of Dh3,000. I would like to take up another job because of bullying and harassment at the office. I hold a Master's degree in computer application. What is the salary limit for me to avoid a labour ban? While I joined my company, I paid for my visa. Will I get that money back? Can I claim it if I don't get it when I quit my job?
It is understood that your employment is subject to provisions of the Federal Law No 8 of 1980 on the Regulation of Labour Relations.
Pursuant to the first part of your question it may be noted that, after termination of your present employment contract, you may avoid the imposition of an employment ban, if you are subsequently offered a salary in accordance with your professional qualifications. The salaries corresponding to one's educational qualifications are prescribed under the provisions of Article 4 of the Ministerial Order No. 1186 of 2010 on "Rules and Conditions of Granting a New Work Permit to an Employee after Termination of the Work Relationship in Order to Move from One Establishment to Another" (the "Ministerial Order") which states:
"As an exception to the provision of the Item No (2) of Article 2 of this Resolution, the Ministry of Labour may issue a work permit to an employee without requiring the two-year period in the event that the employee is starting his new position at the first, second or third professional levels after fulfilling the conditions for joining any of these levels according to the rules in force at the ministry, and provided that his new wage is not less than Dh12,000 at the first professional level, Dh7,000 at the second professional level and Dh5,000 at the third professional level..."
You have mentioned that you hold a Masters' degree in Computer Applications and it is presumed that your degree certificate is duly fully legalised. Pursuant to this and in view of the above provision of law, you should be offered a minimum monthly salary of Dh12,000 in your subsequent employment to lift an employment ban if such a ban gets imposed on you.
Pursuant to the second part your question, it may be noted that employers in the UAE are prohibited to charge the cost of obtaining an employment visa to their employees. Owing to this, you may claim the amount you paid for the cost of visa from your employer should you have sufficient proof of such payment.
You may also contact the Ministry of Labour for any employment related problems that you may be facing in your place of employment.