Bad idea, that won't work. E.g. you've got salary €100 which was $200 at that moment. Next day you spend €100 for a food, but rate has dropped and it's only $100 now. If we followed your ideas, the app would show us that we still have $100 on hand, which is obviously not true. Calculation ($200 - $100 = $100) has no sense at all. You can't compare amounts using different exchange rates, you must always use the same rate.