I got this idea that is a abstract idea that a loan / mortgage. It can be used as a account; this mortgage can be linked to a bill for paying your monthly interest plus extras.

So abstract; a new kind of account:

• Percentage of loan
• Start and end date
• Period payback (can be linked to bill?)
• Calculate extra payment; what will your monthly (or the selected period) be.

Let me know what you think of it.