Did you have a written contract with the breeder? If so, bring that contract and any other details you have (medical bills, emails or correspondence with the breeder, any ads from the breeder) to an attorney who does breach of contract cases or consumer law.
In addition to a possible breach of contract claim, you may also have a claim against the breeder under the Deceptive Trade Practices Act (DTPA) for false statements about the dog's provenance.
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It appears from a quick study that Texas is one of those few states left without a puppy lemon law to help you. I also could not tell about the puppy age laws for Texas. In many other states a breeder or store is not allowed to sell puppy before it is at least 8 weeks old.
If you have a written contract, have a lawyer review it and draft a demand letter to at least try to get your vet bills paid by the breeder. If oral contracts are binding in Texas like they are in Oregon, you will have the same opportunity. I would contact a lawyer about this. It does not have to be a specific Animal Law practicing lawyer because it could be a breach of contract which most civil lawyers can handle for you. Best of luck!
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You have several issues: first, what does the contract say about returns and/or refunds; second, if your vet said your dog is too young to be diagnosed, how would you expect the breeder to be able to diagnose a problem? Particularly a problem that is not hereditary; third, you say the dog is not registered, but then you say you did receive the registration papers; fourth, is the breeder located in Texas?
If the breeder is not located in Texas you will have to sue her in the state in which she resides. That is going to cost you a lot of time and money. Are you willing to give the puppy back? In most states, even those, like Texas, that do not have a so-called "puppy lemon law", require that you return the puppy for a refund OR you receive reimbursement for vet expenses equal to 50 percent of the purchase price.
You should speak with an attorney regarding your options. Your local and state bar associations can give you a referral.
If we do not have a signed fee agreement I am not your attorney and this is not legal advice.