Of the many oxen at a given point in time in the future, which one did he buy?
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.
Here's the new thread for posting quotes, with the usual rules: