Be certain to ensure the tractor does not have a lien, if so, be certain that it's cleared. This is a tough one. Not certain on best course of action.
You had over cash, they give you machine, if it's got a lien and the prior owner keeps your money and does not payoff, I'm thinking lender may come and take your machine.
Option would be to make check out directly to lender for balance owed to lender and mail directly to lender???
Joel