
Hi
I am someone who had the same problem, but I was the buyer, in our case though it was different, we searched for a puppy impressing on the breeder that it must be healthy, as we had a dog with bone cancer and just lost a three year old lab to acute leukaemia, she assured us the puppy had passed all vets tests, sadly she lied on speaking to her vet when we discovered our pup had a grade 5 heart murmur we were told that he had told her about the condition and advised not to sell the pup or if she did she must warn the purchaser, sadly she did none of this. Long story short, she did eventually refund the purchase price, we had no intention at this stage of giving the pup back we knew she would not do treatment and we already loved her, we did go to cardiologist, we had the 6 weeks free kennel club insurance which we carried on with, kennel club would not pay out as she was born with the condition operation and cardiologist fees came to over £5000, she had to have the op at 12 weeks old, thankfully she is now nearly 2 she still has a bad murmur, but you would never know to look at her she is full of energy and fiesty. her yearly check up costs around £400 at the queen mother hospital for small animals cardiologist department and that is just a scan and 5 min chat with cardiologist! It sadly is not cheap. I would personally give a refund, the pup is going to cost alot of money, if you have it back you will either have to pay for pts or do treatment both of which will cost. But that is just my opinion. As I have said your case is different you did not knowingly sell a sick puppy unlike our breeder. And I am not a breeder just someone who tried so hard to get a healthy puppy and still it went wrong lol, life is like that sometimes.
Quality of life, we were told in our case (different things can cause heart murmur) her valves were too small, and her heart was already enlarged one side, without treatment she would not make a year and it would not be good for her, with the operation baloon angioplasty, she would have a chance, she is now nearly two, is never still always running and jumping is in the sea every day lives life to the full, we have no idea if she will make 7 or 8 or live to old age, but she is living a wonderful life right now, we would not have chosen a sick dog but once you have them they are part of the family we had only had her 3 days when we found out about her heart but we could no way have given her back she was already loved.
good luck hope you find a way that all sides are happy