My question involves landlord-tenant law in the State of: Florida

My tenant informed me today via text message that she is planning on moving out. Her lease is up July 2012. She stated she will give me a written notice when she pays February's rent. She said she isn't worried about the security deposit,but she plans to move out end of February and then she is moving back in with her husband ( they had separated). The only reason I rent this house is because I couldn't sell it when we moved and I am paying a mortgage on it. In the end I only make $100 on it. I know by law she is responsible for payment until her lease is up or until I find another tenant. What I'd like to know if it's fair that I ask her for an additional months's rent, in agreement for her to move out? If I did that I would not make her responsible until the lease is up. To me it's a good deal, because she would be released of possibly owing $4000. The monthly rent is $850. Thank you