I hope your day is going well so far. Since you're wanting to pay a bill for a vendor, then I recommend looking into the multi-currency feature within QuickBooks Online. This feature is used for owners who have customers, vendors, or bank accounts that use foreign currencies. You can check out the link above to see if turning on the multi-currency option would be best for your business. If you've decided to continue with this option, here's how to turn it on:
Go to the Gear icon in the top right-hand corner.
Under "Your Company," pick Account and Settings.
Select Advanced on the left-hand side of the screen.
Press the Edit (Pencil) icon in the Currency section and choose Home Currency.
Choose the Multicurrency checkbox and confirm that you understand that you can't undo this feature.