Your letter will have no standing if you take the matter to the labour court or the Sharia court. They recognize only the "legalized" contract. Since the management of your company know this very well, they can do anything with the job offer letter. Consider yourself lucky they are even paying you a month's salary. If they decided not to, you will not be able to do anything to them. Just silently take whatever you are getting, thank them with a smile on your face (!) --and leave.