Trying to **** me for one extra month rent - By delaying credit checks

I have been trying to end my rental contract with the Polokwane branch since the 29th of July. I would like to move out on the 31st of August. I found a replacement for my apartment. I went to their offices with my replacement last week 12 August. We spoke to the agent (Marilet) and she agreed to assist with the paperwork, however a week later she hasn't done anything and she is busy delaying and telling me I will be liable for the September rent while she is not doing her credit checks on the new person who submitted her documents. This woman is very lazy to do her work and is trying to **** me for extra rent. It didn't take her more than two days to finalise my documents when I moved in because the place was vacant, now its taking time?