And nothing would actually force them to pay up.
If you had enough realms borrowing funds from you, then you could offer some amount off the principle if they attack the realm that decides it doesn't need to pay.
Or you will just have to have a significant army of your own. Besides, having a bank on your side would be a rather good thing if you need to get involved in any conflicts and braking that tie would cause some incentive for the bank to fund your enemies.
In other words, no. there is no official way to make them pay up, but if you get all godfather about it you could make it work. (Thinking about an army of infiltrators
)
