I appreciate your time and informative replies.

However, I don't think I was trying to modify the contract, that is the same version of the lease I have sent them at least three times before the end of the lease. And I know he can sue me if I deduct the deposit as well as I can deduct the deposit because of the damage to the house caused by them if he refuses to sign the addendum.

But I have to admit that I don't have any legal backgrounds/knowledge and what I have written here are based on my own thoughts, that is why I am here and try to learn. Anyway, I do apprecitate your time, I did learn something from you today.