It’s not that money is enforceable and payment in non-liquid moveables isn’t…
The point is that transfer in title of money happens when someone is paid. It would be absurd to talk about money having changed hands from Chester to Beatrice if it’s still sitting in Chester’s cash register, even if he had promised to pay it to Beatrice in the near future. If that money is stolen from the cash register before he pays her, the thief is stealing Chester’s money - not Beatrice. Chester is still liable to Beatrice in terms of his personal promise.
Chester must make it such that Beatrice can take delivery of the money, in legal terms. He must ‘pay’ her. That’s the moment at which title transfer takes place. If the thief steals it after this moment, he is liable to Beatrice, and Chester is no longer liable to her.
With a non-liquid moveable, transfer in title can similarly only take place once the old owner performs in every way necessary to make the thing available for delivery to the new owner. If it’s absurb to talk about money in X’s cash register/wallet/savings account as being held in title by Y, then I feel that it is absurd to talk of X’s heart/liver being held in title by Y while it is still in the body of X.
Otherwise you get this…
Imagine that X promises to allow Y to cut off X’s left pinky finger if X defaults on the primary agreement. X defaults on the primary agreement, and is now liable to allow Y to cut off his left pinky finger. Y justifies the force necessary to achieve this by arguing that title in said finger was transferred to him the moment X defaulted on the primary agreement, and that the finger is therefore his.
What if a third party - Z - cuts off X’s finger after X defaults on the primary agreement, but before Y is able to enforce the secondary one? I suppose one might argue that this achieves the implicitly punitive purpose of Y, but I feel that this is besides the point, as the purpose of private justice is fundamentally compensatory and not punitive or pedagogical. What if Z was doing this in terms of a similar agreement he’d had with X? Don’t you then have a situation in which X can enter into an infinite number of contracts in terms of which he offers the same left pinky finger as security?
In any case, is Z now liable to provide Y with X’s severed finger? What if he loses the finger, or destroys it? Is he then liable to provide one of his own fingers?
All of this absurdity is unnecessary if you allow that transfer of title cannot simply be a matter of personal agreement contemplating conditional perfomative outcomes. There must be a real, single moment at which title transfer takes place - it can’t be an uncertain contingency.