It is generally better to save the balance. The general rule in
accounting systems design is "what can be printed, should be
explicitly on disk". for an invoice:
value before tax, tax percentage, value after tax, total before tax,
total after tax, etc, should all be saved explicitly.
An account should have a balance. Every operation should have balance
before operation, value, balance after operation. You should never
update an operation.
This way when business rules change all previous documents are stored
in consistent state.
Greetings
Marcin Mańk
