My vet overdosed my dog on amitriptyline....?

I went to the vet when my dog had an itchy rash on his belly. I was given a bottle of medicine and told to give it to my dog twice a day. After 10 days my dog had no relief from the itchyness, so I took him back to the vet. They looked in the bottle and realized they had put the wrong pills in the bottle. Instead of prednisone, amytriptyline was in the bottle. At the dosage of 2 a day, my dog had been ingesting a double dose of doggy antidepressants for 10 days. They flushed his little body with fluids and checked his blood. At that time they said he was fine, but he later started having seizures. Now my dog has to take medication for the rest of his life (phenobarbatol twice a day) to stop the seizures. My vet makes ME pay for these meds!!!! He did give me a one month supply for free, but I have been paying for it ever since, because they say, he might just have epilepsy, which is hereditary. His parents and his 8 puppies have no signs of epilepsy or seizures. ADVISE?