Urgent question on Deposit

[Jak Dadi]

I had a potential tenant submit an application on 3/16. We approved their application on 3/19. They indicated they wanted to move in on April 1. We met on 3/23 and they gave a money order of $500 to hold the place. I gave them a receipt. They said that they will pay the balance and first month's rent on April 1st when they move in. On 1st, they indicate that they will sign lease on 4th. On 4th, when I meet them, the gentlemen asks for extended time for security deposit. That was the first he speaks of needing time for deposit. His girl friend would help him with the balance deposit of $200. His rent is $900 and deposit was $1000. I give him the keys and a weeks time for deposit. I have yet to give him the executed copy of the lease since I did not get a chance to make a copy of the lease. On the 6th, he asks for the lease since he wants to start services. Meanwhile I noticed he had not given me copy of proof of income. So, I indicate that I need that, in addition to balance security deposit. I express my concern that if he has trouble with deposit and I have not seen proof of income, how can i be comfy that he can pay rent.
Finally, we decide to call the whole thing off. The question is how do I treat the initial $500. Since I stopped showing the apartment and I lost rental revenue, how much I can keep, for how many days?