I am not sure about the validity of contract but no, you don't need any document from your current employer for H1B transfer. Try to get reference letter for future need. You need this. As far as I know once the contract is expired what is there to do with release letter! I'm sure you don't need that. By the way keep it to yourself till you get H1b transfered. Best of luck. By the way what you are working as let me know.