Can magnet placed beside cartridge damage a coil?

I received a 20 year old NOS cart from a dealer which I resold as new but ended up having to refund the payment back to the buyer after he found it to have a channel imbalance issue.

I sent it back to the dealer to check out and he says there is a large difference in coil readings of 1.3 (normal) vs. .8. He says he tested his entire stash of these old carts prior to selling them and says the only explanation is that somehow a magnet was placed close to the cart either during transport or by the person I sold it to and thereby damaging the one coil. He therefore assumes no responsibility to replace it.

Would this be possible?