Company Keeps My Training Certificate

I am working for Customer Service Engineer in Medical field. I have training for 2 week over sea training (feb 2011) and bond for two year. My employer ask me to give one week for my certificate for ISO purpose. Now say will not give back my certificate until i resign or end of contract. Contract will end 2011 Dec 31. Employer can keep my certificate?

It is not a good gesture, but if the company has paid for the courses, why cannot?