Seller doesn't move out on time

[Sasha Tetera]

We are in the contract with a senior citizen who has asked 30 days to close so she can move out. We followed up through the month and she assured me that she will move out on time. We are due to close in 3 days (September 1st), however, her new place fell through and her now she is asking for a move out date of September 28th. 

Gladly we put the provision in the contract about her potentially staying till October, but what would happen is she doesn't move out by then? 

Here is exact verbiage we put in the contract: "Any damage to home prior to move out is the responsibility of the seller/tenant. Title to hold $500 security deposit. Rent Back Agreement: Seller is to vacate the premises no later than September 1st. Daily rent to be collected by the title company is $25 per day. Occupancy limited to October 1st" 

While I am respectful of her situation and willing to work with her, I also didn't expect to find myself in this situation. I don't want to lose leverage and second-guess if I should closing before she moves out. What are the implications if I do? 

Thank you!