They were storing balances in kobos. 😆
I asked, "If a customer has 1 trillion, how will you represent it in the database?"
That becomes 100 trillion kobos.
Then comes the real question: How are you handling overflow?
I once reviewed a code of engineers working with one of the outsourcing companies in Nigeria.
On a Banking Back-End, They checked the balance, and then they credited the receiver before coming back to debit.
If your Payment App is not designed like this, you are wrong. 👌