The bride and groom really just want cash, but just stuffing a bill into a card doesn't seem very festive. Not that anyone complains about a crisp green bill falling out of a card. I remember a birthday party for a very young nephew; while opening his first gift, a card, he announced, "What, no money"?!
I found this lovely card at The Croft Studio on the Isle of Skye in Scotland - perfect since the groom can trace his ancestors to Scotland.
I saw the money heart on Pinterest - detailed instructions here. Since I am not an expert at precise folding , I did a couple of practice ones with dollar bills so the crisp gift bill wouldn't have all the "do-over" folds! It was much easier to fold the new bill as the crisp paper held the creases better. You could make this with patterned paper as an embellishment- fun for a valentine card. After the heart was folded I weighted it down with heavy books to flatten it.
Moolah with style!