Foreign Exchange Purchase Order in US working in AUD

Hi I have set up main Account as AUD, set one account payable in $US, set forex rate as paypal exchange rate. When I come to spend money it seems the ammount put is in AUD, but account for the invoice is in US… and then ammount unpaid is the Difference in the US/AUS… Not sure if this is a bug or I am doing something wrong… I dont see much in the guides?

Total 18.45 Amount paid 13.44
Balance due $ 5.01

I paid 18.45
This is the amount paid on the purchase invoice, if I edit it it shows the invoice as $US in the greyed out area at the end…

It looks like when I pay something I will have to put in AUD equivalent to the US figure, but then what is the point of having the account in US (for that supplier) …Is there a way around this? I want to put in the US dollar ammount with the PAYPAL ex rate on that day… I guess I could just add in the AUS in payment?
It has 18.45 AUD in accounts payable when I entered that as US amount in purchase invoices, and says US on that invoice, maybe a bug?

Actually I checked if I did a purchase invoice and spend money in today’s date, it worked if I went back in time to add in previous time (adjusted forex rate), it was that large difference out?
thanks

If you paying for the purchase at the time of the purchase - then just use AU$, no need to use US$
You would only use both currencies if there is a time delay between the purchase and the payment.