It is not a security deposit if it is rent you are collecting. If they want to pre-pay there is no law to stop that. Just make sure that you give him a receipt so it is clear this is what has been done. Also get a SD from him as well though. If he passes the credit and background check I don't see anything wrong with this. I have read where people who sell drugs sometimes do this because they come into a large amount of cash at one time but if he has a legit job then there should be no trouble.