Wouldn't it be great if ...

a) If the renter wants to cancel before it ends, the renter ends up paying for the full rent; and

b) If the owner wants to cancel before it ends, perhaps the card should be locked as "out of use" for future rentals until the original rental time expires. e.g. Player A rents for 20 days. Owner cancels it after 5 days. Owner cannot relist the card for rent for another 15 days.

That would stop owners cancelling cards just to rent out at higher prices to someone else. But it would still mean that an owner could call back their card if they wanted to use it to play with in their pack.