But in a smart contract, that gets executed automatically.   If he takes away a million because I am contractually obliged in any case, that's not theft.  They are his, now, even if I keep them (unlawfully now) physically in my possession.

If you OWE me 10 dollars, then I can rightfully come and take them.  It is just a matter of politeness if I ask you.  They are mine.

If I buy a house, and I pay you, then now the house is mine.  I don't have to beg you for the key now.  It is now legally mine.  I can politely ask you to hand over the key, but if you don't, I break into the front door of my (now) own house and if you try to stop me, YOU're the wrong guy.