Doug, you are correct.
My client did not explain to me well enough his situation.
He kept telling me that this was a discount offered by the Supplier, without touching the HST/GST tax.
I showed my contact this thread and your comment, and he finally was able to explain that he was referring to an early-payment discount.
This of course can be handled properly in AP with setting up the proper Term Code.

Thanks for helping me out on this.