tracking exchange rate history

[lsmith77]

in my example, the external system asks sylius for an order number and a total amount in a specific currency.

imo, it is the job of Sylius to calculate the conversion between base currency used for the order and display currency at the rate available when the order was completed. 

for example: if I use EuropeanBank Interbank FX in Sylius and my supplier is using PolishBankInterbank FX, by doing the conversion from base currency (RON) to EUR at another conversion rate, would result in a different amount to be paid

when calculating the "amount paid" for display purposes, you need to take into consideration historical FX