1. Get the mis-placed puppy back, as soon as possible.
2. Depending on for how long the puppy had been away from me, I might give a full refund, but minus a minimal boarding fee in the event it takes a while finding a new permanent home. But don't do this UNITL you have the puppy back or, Return Clause or no, they might decide to sell your puppy elsewhere!!
Happily only once when I sold a slightly older puppy we'd kept to show until the arrival of that dam's sister's litter which had 2 better puppies for me to keep, going forward, did she bounce. In fact, hearing that their Squaddie son had come home and was kicking a football AT her, making her terrified, we went straight there and collected her. Saying I'd make a partial refund (we'd only charged a minimal amount as she was older), once we'd found another home, so minus Boarding Fee. They seemed ok with that but got back to me later on saying they wished they'd rehomed her as they could have asked more for her!! Clearly somebody had been talking to them.
I don't understand how, if your puppy was only picked up today, these new people could have decided their Chi wasn't getting along with the puppy! Returning it the same day because of that ..... odd. But GET HER BACK.
All this is the down-side of breeding I'm afraid.