What to do when a tenant vacates in the middle of a deal?

[Conrad Martin]

If you had a property locked up that was cash flowing, and the renter leaves after the contract is signed, is that grounds to re-negotiate the offer? Especially if it was negotiated with the turnkey status as a feature of the property? Does this qualify as a change in the condition of the property? Can you leverage the vacancy as a loss in value? 

Asking for a friend ;)