Whenever we get a thread on buying a used tractor, or selling a used tractor that was financed through Kubota the solid advice is this. Caveat Emptor. Buyer Beware. Smart buyers would never just hand over the cash without first asking for the original loan paperwork, on which is the serial number and loan number. A smart buyer goes completely around the seller and calls Kubota directly. No monkey business would ever occur.
In the end, the true seller of a used, financed tractor, in such an instance, is Kubota Credit. They are the true owner for that matter, until their interest is satisfied. The buyer should drop a dime, speak directly with Kubota and hash out the details of the matter. That's how is should be done and IS done every day.
A wise buyer would never be a party to what the OP described, even theoretically, in the opening post. Just wouldn't happen.