I am sorry you got that news from your vet but on the other hand, be glad that it was detected early.
What the other members have posted is correct, a regular vet is rarely qualified to grade a heart murmur. Having detected one, the absolute person to see is a board-certified veterinary cardiologist.
Please make an appointment as soon as you can with a board-certified veterinary cardiologist. AmbikaGR posted the link for ACVIM. I had a very similar situation when my Chance was a puppy and my regular vet pushed me to make the cardioligist appt asap due to high demand and sometimes a wait to get in. We waited 3 weeks for the appointment so do this as soon as you can.
A cardiologist at a veterinary teaching hospital would be great if you can get an appointment with one.
Don't be fooled by an energetic puppy without symptoms. Chance had no symptoms until he was 8 months old yet the cardiologist confirmed a grade 5/6 murmur.
Your breeder sounds awesome having already agreed to help with expenses by refunding your purchase price.
For now, don't get ahead of yourself with worry about the what-ifs. First get your vet's diagnosis confirmed by the cardiologist. Then go from there.
The good news is there are treatments available and diagnosing it early is important before the heart sustains permanent damage. Chance was diagnosed early and treated. He is 7 years old, has yearly a cardiologist checkup (ultrasound,etc) and lives a completely normal Golden life. Does agility and competes in Rally-O. No restrictions.
If you need more info later, I am happy to help. Keep us posted.