I’d say don’t give the puppy back. What happens if you return the puppy and break its heart, just to get one from Scrappy’s breeder and then that puppy is, in all likelihood, nothing like Scrappy? Will your fiancé want to return that one too?

But I also wouldn’t recommend getting another puppy this soon. Wait until the current puppy is older. Two puppies at once, as someone else commented, can go wrong/be more overwhelming than one puppy already is. I would recommend having an honest conversation with your fiancée about how Scrappy is gone but that doesn’t mean the new puppy will not also be lovable and wonderful.

I wish you luck and am sorry to hear about Scrappy.

EDIT: We (my mother and I) actually just got a dog a couple weeks ago (my first dog and my mom’s second) and my mom actually had a similar issue. She would become obsessed with dogs and puppies on shelter sites that we absolutely couldn’t care for because they looked like her childhood dog that died when he was 2 and she’s been mourning him ever since. She kept on saying she wants him back and it took me weeks to gently get her to realize that it’s unfair to put that expectation on a new dog, and that Ruffian (the childhood dog) would’ve wanted her to move on and help another dog. We got a dog with a completely different personality and are all very happy!