Open Question: How to make a commitment to the person who I am subletting to?

hi...I am subletting my apt for a month, and there is a couple from France who want to take over my place. However, we have problem in making a commitment. I want them to pay the half month security deposit before his arrival, but he told me last time before he came to New York city he paid the deposit while the person who offered the housing disappeared and he never got his money back. And it is also very often to see that someone who promises to take over the place while leaves or changes mind at the last minute without noticing. I am curious if anyone who has any suggestion that if there is a way that can make him believe I will return the money, and make me believe that he will take over the place as he promised. By the way I really don't wanna send him the copy of my lease since I think it is very personal. Thanks so much!