Yea, in this analogy you already own the tractor, so you already own the tractor's money. I don't think I'm going on a limb by saying that isn't slavery. You already own the tractor and the money. There is no emancipation needed.

If somebody else's tractor approached you with a smart contract, the owner of that tractor would have a case that the smart contract was not entered into in good faith, since a tractor cannot enter a contract at all. Maybe you'd even get charged with theft.