feeling rather dumb at the moment

Reconciling DF's business accounts atm and there are some PayPal transactions for overseas purchases. My dilemma is that there was USD money in his PayPal account so PayPal didn't need to convert AUD to USD before sending the funds, the thing is I need to convert it so I know how much he paid in AUD but PayPal only says the USD amount.

How do I convert the amounts? I can't use an online currency converter as I have the old exchange rate for that day so can't use the exchange rate for today as it's significantly different.

I hope that makes sense..please help a math's dummy