I don't know what state you live in, but, i am in the same situation and live in CA. You don't have to give the puppy back; according to the CA's puppy lemon law; you have three options, one of which is to keep the puppy and get reimbursements for all medical related expenses to the congenital defect, which PL is. here's the link to the CA law
Dog-Play: Getting a Dog Tips - California's Puppy Lemon Law