Oh. I see what you mean.
He bought the ones on the future side of that worldline.
It's convenient that way, and humans are good at keeping track. He could have bought any combination of future oxen the guy owns. This has the advantage of later oxen being in the same area as earlier oxen, simplifying transportation.
I'm not making myself clear. It's clear from what you say that if Heraclites bought an ox yesterday, he owns an ox today. But in order to say that he owns this particular ox, he needs a better system of identity than "you never step into the same river twice".
Here's the new thread for posting quotes, with the usual rules: