I'd want a copy of the Pre-lims, for my vet to examine, you need the documentation. Still if you have it in your contract result at 24mt. you don't have to accept pre-lims, that's not your contract. But as he's so non-chalant about putting a 14mt dog down, that he's invested so much money in, perhaps has a vet agreeing to it, and doesn't want to wait, it seems like something else is going on. It doesn't make any since unless the dog is crippled or having other health issues, associated with the hips poor rating. Doesn't seem like an owner I want any of my pups with, You might want to get the dog back, for that reason alone. If you do refund the money the dogs is yours, he cannot put it down, still I'm wondering about her condition with a situation like this.
I have seen a dog come back good-fair after a semi-dys prelim, I've seen them come back good on a very large dog, after the Orto-vet told me to wait another year and send them in at 3. Dogs particularly big dogs grow slower, hips and bone structure can still be changing, (there's a reason they are called Pre-lims)dogs are still growing, if the rating couldn't change they'd just be Lims & OFA wouldn't make you wait until 2. Let's not forget bad positioning, wrong interpretations etc.