A user has a balance which starts at 0 on user creation and reflects the sum of the amount of all cashflows.
Each cashflow has an amount, a description, a timestamp, and the user it's assigned to.
A credit is a positive cashflow amount. (just put 1000 etc)
A debit is a negative cashflow amount. (just put -100 etc)
You can use this to build a bank or any other finance/account system. You can also study this architecture to include in your own rails apps.
This is super basic without any ui frameworks to just give you a boilerplate starting point where everything works but no design customizations have been made.
Dear Noobs: remember to rails db:setup
after you clone it.