In GP 2010, I have seen instances in one of our foreign GP companies that the sales invoice would post in US Dollars. But, the resulting journal entry would post in British Pounds.
I did some testing in Fabrikam and created a sales transaction in Canadian Dollars. Upon posting this transaction, the journal entry remained in Canadian Dollars. I say "remained", because GP does not let you change the data within the "Currency ID" field.
So, how is it possible that--in our production environment--someone posted a journal entry in one currency even though the subledger's currency was different?