In the company setup options, there is an option for "Calculate tax rebates' which is used with payment terms. With this option on, the payment terms discount will correct the tax amount with the discount as well. This only works with functional currency. Our ask is that it also works with Multi-currency.

If the invoice is in an originating currency, it will not post and fails validation. It says the discount available and discount taken amounts do not equal the actual amount. It's like it can't factor in the amount of the tax rebate with the discount to pass the validation. Perhaps many companies didn't use MC in the past so this was never coded, but MC is pretty much required now, so this company setup option should work with invoices in originating currencies too.