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?

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.