commodity $
    value 10 EUR

commodity USD
    alias FOO
    value 25 EUR

2012-03-06 KFC
    Expenses:Food                $20.00
    Assets:Cash

2012-03-08 KFC
    Expenses:Food            USD 750,00
    Assets:Cash

2012-03-10 KFC
    Expenses:Food            USD 750,00
    Assets:Cash

$ hledger -f- check

#test reg food -X EUR --now=2012-03-15
#12-Mar-06 KFC                   Expenses:Food               200 EUR      200 EUR
#12-Mar-08 KFC                   Expenses:Food             18750 EUR    18950 EUR
#12-Mar-10 KFC                   Expenses:Food             18750 EUR    37700 EUR
#end test
#