If I start with 1000 tokens I distribute and 1000 dollars in my vault, no matter what you do with the tokens, I can exchange a token for a dollar — because your manipulation of the tokens doesn’t remove dollars from the vault. Those only change when: 1. I receive a new dollar, so issue a token; or 2. I destroy a token and release a dollar.

“Shorting” doesn’t impact that: if you borrow someone’s ticket, then sell it for $0.85, that doesn’t create a new ticket — there is still one ticket and one beer… and one IOU. What happens to the original owner is either the borrower buys a ticket back and returns that (canceling out the IOU) or else that original owner no longer has a ticket — they’re owed the value of a ticket by the borrower.

In the case of tokens/dollars, that value is easy to assess: the person borrowing your stable coin token who fails to return it owes you $1… but that doesn’t come out of my vault, because you don’t own a token. You’ll have to get that $1 by suing the borrower over failure to deliver.