I'm not entirely sure what you mean by "the amount would be shown" but if you want to know an amount when the transaction actually happened all you need to do i

I'm not entirely sure what you mean by "the amount would be shown" but if you want to know an amount when the transaction actually happened all you need to do is call the API beancount.core.inventory.Inventory.reduce(position, currency, price_map, date) and pass in the date you want. Though I agree with your complaint that beancount surfaces it in inconsistent ways. If I change today's exchange rate for USD/GBP then many parts of beancount will then (incorrectly) change my income for the year 2015. But that is really a bug that could be fixed (using the API I just highlighted) rather than some fundamental problem. And surely on hackernews one should hack on software and improve it.