If you make your opponent play Change on a Possession turn, what happens? Do you get the debt? This is a blue-dog question, and I assume "the difference" refers to the difference between the card you trashed and the card you gained, meaning that you don't get the debt.
I'm going with, the gain didn't happen, so there's no debt.
But there’s no conditional?
What I would read it as would be:
If the possessed player has no debt, you possession-trash a card; its selected, when it tries to end up in the trash, it’s set aside, to be discarded. Since you are on the second conditional, you possession-gain a card; that is, it’s gained to you instead. You compare the gained card with the card, now set aside - and possession-gain the relevant debt?