My company found a work around although you still need to calculate the discount on your own:

We created a GL#, 85115 A/P Discounts Earned, that I enter on the second line of the category details section and when you put the discount amount in the amount box you make that number a negative. Of course you still need to enter the regular payment info and GL# on line 1 of the category section

It saves from at least having to create a separate credit voucher to take a discount.