My question involves a roommate in the State of: Arizona

I am the only person on the lease and i have four other guys living in the house with me. At the beginning of the one-year lease we all split the $2,000 security deposit ($400 each). The lease is up in three months and i am going to move to a smaller place. One of my roommates, Bill, moved out at the end of Nov. He let us all know a month in advance but didn't do anything to replace himself. Luckily i got someone, Tom, to fill the room for 3 months. Bill wants Tom to pay him the entire $400 share of the deposit. Tom agreed to pay him $100 because he is only going to be there for a quarter of the lease term. Bill would get the remaining $300 in Feb. when the lease is out and we get the deposit back. Bill doesn't like that idea at all. I think that is fair because Tom should not be held resposible for any damages that occured during the 9 months he didn't live there. What do you think? What legal and/or moral obligations do I have? Thanks in advance!