It's a different look when the icing sheet is applied directly to the cake. So it depends on the look you want.
If you want the bills to look raised and a bit wavy, them cut the rectangles out of gumpaste, let them dry, paint the sides with green, and apply the icing sheet bill.
If you want a "flat straight" look, you can apply to the fondant.
I didn't see the cake, but the first way on gumpaste seems better for $100 bills.