回複：What should I do?

I am moving out from my apartment. My roommate returned the security deposit and half of the unused rent to me. But she is keeping the other half, saying that is to protect herself in case she couldn't find a new roommate. There is no written or oral agreement prior. She just kept my rent money after I found a place to move out. She said if no one move in this month, I will loose that half. If someone does moves in, say in the middle of the month, then she will give me half the money back. She said it is a way for she and I both share the risk. But she is the only person on the leasing agreement. I believe she should be responsible for all the risk. There is no receipt for the transaction. What should I do?