Pro-Rated Rent Dispute

[Erin Shirey]

I purchased a pair of duplexes in January, and one of the tenants moved out yesterday, with her lease ending today. She did not send any rent for August, so when she asked if she was good to get the full deposit back I said that I would have to deduct the pro-rated rent for August, but otherwise things looked good. 

She says that last year, even though they moved in on the 11th, they paid the former landlord for the whole month of August, and therefore they should not pay any rent this August.

I have an estoppel agreement signed by the former landlord, the tenant, and myself, in January, saying there were no pre-payments associated with her lease.  I have suggested she contact the former landlord to work it out with him but she wants me to call her to "discuss it further".

Am I ok here? Is there anything I need to do to be protected?