Hello Sam,
I seem to remember that somewhere in Adagio you can print a credit amount as a negative number, but I cannot find it at the moment, and I don't think it was the discount amount field in any case.
Since the Forms Designer does not support calculations, there is no way to do this unless we specifically allow the option or create a field with the sign flipped.
I'll see which is easier (although I'm sure R&D will say that "Less Discount:" should be clear enough!

)
You'll be able to lobby in person in a couple of weeks. See you soon.