In the first case, you only know the AMOUNT. You can't undo transactions because you only have 1 object stored, the total amount. If you store DIFFERENCE, then you'll have a table of every transaction and can undo them individually. The cost is having to sum the table whenever you want to know the amount (which is usually worth the flexibility).
Regarding the performance - you can add triggers writing to additional table with a sum, or maintain that table in the code that adds rows, or calculate it each time you want to know the sum.
