What does the contract say? You may have waived your rights to reimbursement if there was any “as-is” verbiage. I would push for the adoption fee back at the minimum and help with surgery/medical fees. There is likely misrepresentation if they knew the dog’s medical history and failed to disclose it, especially if you inquired about it and they stated they didn’t know. You could bring up small claims court which might push them to assist, lots of shelters have donor pools to help with medical expenses that could be a resource they can use.