"Please advice if anyone can have any idea/suggestion to show these people that it shouldn't be this easy to promise their customer and not keeping it"
Believe me, Its very easy for them to do this even if you had anything in written. Unfortunately Real estate laws here in Qatar are not very strong. You can always go to the Police and try explaining them, but, agents will find a way through anyhow. Despite being smart, i have lost my money twice to real estate agents. The only thing that we expats can do is find a reputed real estate company and rent home from them.
Small time real estate free agents dont really care about reputation and law.
My Suggestion - Give 10% Security Deposit and leave. This is how Market operates here.