Proper way to cancel a Contract for deed/mortgage?

[Keith Lewis]

Recently found a property with owner financing. The owner sold the property to an individual and had a contract for deed. They ended up getting divorced. The wife hasn't paid him in 6 months and he offered to sell it to me if I could get her out (he is so CHEAP!) 

I negotiated and offered the tenant cash for keys. They agreed to my terms and will be leaving in January. Now, I already agreed on a price, downpayment & contract is signed. How should the owner cancel the contract for deed? Since they left in a cash for keys scenario how should this be canceled properly so I can hold a clear title?