Bank account balance data structure design and code
I think using dictionaries is about right for this small project.

However, the code will not scale well in terms of readability with lists of tuples. You should consider using object oriented approach, and implement some serialization, deserialization methods (for storing the data onto the disk and reading it back). That way you can add more functionalities in a more manageable way.

I don't understand why you allow negative balance!