1st , Last, Security - basic questions

[rent_rage]

My girlfriend and I rented an apartment by basically giving the LL a check for one month rent to hold the place and it was my understanding that when we moved in July 1st we would give him another one month of rent for July and the first check would be applied to the last month of rent, which in our minds was his 'deposit'.

Now he is saying that we agreed to a 'first, last and security' and we owe him another month's rent.

There were no lease signed when we gave him the first check [the receipt was scrawled out on a piece of paper] because he still needed to 'get the paperwork together' for when we moved in. He finally got it to us a few week after we moved in but never pressed us so we still haven't signed. I realize this little tidbit of info keeps us in the clear, but I don't really want to move again right now [too busy], so I have a few questions:

1. If we say 'no way' to the 'security' he wants [on top of our last month we already paid], how long does he has to give if he wants us out?

2. If I give him a 'security deposit' check, can he cash it before the end of the lease?

I know these are kind of basic questions, but I'm new to dealing with slumlords......guess I've just been lucky up to this point.