"A magnetic field exists as long as the magnet (qua magnet) exists, but it is not a property of the magnet."
Sadly, this simple example can be attacked in myriad ways: What are magnetic fields? Is it something that really exists? It seems rather like a mental abstraction to describe electromagnetic interaction.
In general there is probably always some ontology, in which we can attack a sentence of the form:
"X exists as long as Y, but is not a property of Y."
Still, what would be a better example than X = magnetic-field, Y = magnet?
Could you give me an example, that is not so obviously flawed?
A few clarifications:
there is a lot of wiggle room how to understand "as long". For example, if something happens to the sun, we won't notice it for minutes. So "as long" doesn't have to mean exactly for the same time period.
the connection has to be strongly scientifically validated. There shouldn't be any conceivable (in the sense "not violating known laws of nature", not in the "metaphysically possible" sense) scenarios, where the connection doesn't hold.
admittedly "property" is a bit vague. But it definitely shouldn't be a case of supervenience, like the life a cat depends on its body.
the example doesn't have to accommodate every fringe ontology.