Should i trust my employer?

Here is my case:
I am going on vacation with my family for one month this December 22 till January 22. All is pack and ready to go. One thing bothers me is the action taken by my employer. Today, December 20, 2008 they required me to signed a paper that states the cancellation of visa and will serve as my resignation also.

At first, I am very hesitant to sign it because I know of the consequences if I did. That I will give them full rights to cancel my working visa which will be valid till August 8, 2009 even if I am on vacation.

I explained my side that it will be unfair for me because I can't do anything if they cancel me for I am outside the country not to mention the things I will leave behind and the graduatee for working on almost five years.

But they said that the paper that I signed will only be submitted and used to the ministry if I will not go back after 6 months. They said also that this is a new policy that the company will implement to any employee going on vacation.

Do they have the power to cancel my visa even if I am not here in Qatar for just a month because I signed the cancellation paper?

Should I trust the company and believe that they will only use it for precautionary measures?

Does the rules they are implementing is legal?

I really need your advise so that I can go to Qatar Labor Law before I leave Qatar. I flight is Dec 22.

Thanks in advance.