Per-transaction currency

[bennypowers]

Say I charged my ILS card with a USD transaction, something which is quite common here. rather than creating duplicate accounts per currency and manually setting up the transfer, It would be helpful to have a per-transaction currency field with a conversion rate (automatically populated from market data)

[cklamra]

It would also be very useful to store different currencies in one account without converting on adding.
My use case would be a single savings account with assets in different currencies (e.g. 100 EUR and 100 USD); current exchange rates would only be used to show the total balance in the default currency (total: 190 EUR as of today).