Rereading my last post, I realise that whilst correct it may be misleading.
Let me try this your way; You start by saying: Oil is US$100 The A$ is 80c to the US$ OK so far, but having received the goods the buyer transfers US$100 into the sellers bank account in Australia. The bank applies the currency exchange rate, crediting the sellers account with A$80 (not A$125). When the A$ is at parity, the bank will credit A$100 to the sellers account.
