TX Account:(CAD), Vendor:(USD), Payment:(CAD) Posting this mixed-currency bill results in the mixed-currency dialog and all works as expected. Paying the bill results in the same mixed-currency dialog but the exchange date is set to current date - not date of payment. Since the exchange date is greyed out, there is no way to correct this error. Result 1: Currency list is not updated for the date of the actual payment (inconvenient) Result 2: Currency for TODAY is updated with wrong value. As far as I can see, this does not affect the actual values of the transaction so I don't consider this critical.
Thanks for the report. This will be fixed for gnucash 4.5.