I-140 approved under EB-2 category (India), my employer is not handing green card related (I-140) documents to me.

I am a Canadian citizen working on H1B visa as a IT consultant. My employer filled green card for me few years ago, I have I-140 approved under EB-2 category (India), my employer is not handing any Green Card related (I-140) documents to me so that I will be tied up with them and can not switch my employer for years and they keep on making money on me. I have no intention to leave this employer presently but what should I do to keep my priority date locked if I have to change this employer in future. It is very frustrating how some employers misuse their employees.