A more literal loan card:
Loan (b)
Cost $0. Treasure.
Worth $-2. When you play this, trash it.
When you buy this, +$2 and +1 buy. You may not buy multiple loans in one turn.
As written, why would you ever play the card (unless you were in a position where you had $2 more than you needed and actuallly could trash it without penalty? There's nothing to force you to, and better to have a dead card than negative money. Unless you put some form of clause on Loan forcing it to be played when in hand during the buy phase, and forcing everyone to reveal their hand each turn, nobody would ever play one unless they could do it without the penalty mattering, at which point the penalty, isn't.
Maybe an interesting thing to do with the penalty would be to have Loans be worth exponential negative VP (i.e. each loan worth -1 per loan in your deck, like Gardens or whatever) and have you be able to play them as cantrip actions/0-value treasures, with a clause allowing you to 'buy' them during your buy phase to remove them from your deck for more than they cost initially - maybe also make them untrashable any other way.
Something like:
Mortgage
$2 Action- Loan
Worth -1VP per Loan card in your deck at the end of the game (Defining a new card type both to avoid confusion with the name and the type being the same, and to allow for potentially more cards sharing the type)
When you buy this, +$4 and +1 buy
+1 card, +1 action
Whilst this is in play, during your buy phase you may pay $5 and 1 buy to return this to the supply. This card cannot be trashed. If an effect would trash it, instead set it aside and place it in your discard pile during your next cleanup phase.
I dunno, it sounds like it has the potential to work with potentially some playtesting and rebalancing. Useful effect, difficult to get rid of, very nasty consequences if you don't. May need wording on how it interacts with cost-reducers and Contraband.
Another interesting option would be to trade in victory cards to pay it off - MY ESTATES ARE BEING REPOSSESSED!