requestor cannot select currency in PR/ PO line when vendor has been selected
If the requester is selecting an Item on a requisition line, there is very little choice about which vendor they can use... in fact I think the most it will allow them to choose is one of two vendors (the cheapest, and the Primary).
If you can order from Vendor A in HKD and USD, I suggesting creating two Vendor accounts for the vendor with different currencies. And then create a trade agreement price for the item against the vendor account that has the currency you want.
requestor cannot select currency in PR/ PO line when vendor has been selected
Which D365 app are you using? This forum is about D365 in general, while your question seems to be about a particular product (such as Finance and Operations or Business Central).