I did request a refund from him but he refuses to, in any way, do so. Whoo......... hold on, did you ask for a refund and
also to keep the pup too,

if so no breeder would do that! Especially on the word of a vet who may well be jumping the gun over a flow murmur which is becoming more and more common for a vet to worry an owner over and is something that may well naturally go away.
One vet may take no notice of a flow murmur due to their experience others will use it as a way to get insurance money from unsuspecting new owners whilst the going is good, so the breeders vet either did not notice at all or decided not to make an issue of it, were you shown the breeders vet report on your pup?
Is this a breed with heart issues, were both parents health checked? If not that is one issue, if they were checked and the breeder has done all they could sometimes bad luck just happens, depending on the grade of the heart murmur it may well cause the breeder to give you some recompense morally, but if this is a young pup you need to wait and see, of course a breeder will not give a full or partial refund at this stage.
You are perhaps acting on impulse at being frightened by your vet, you should keep communications open with your breeder and not jump to conclusions before a second opinion if need be by the breeders vet and finding out just what type of murmur this is.
It's a little soon to start naming and shaming a breeder who may have done nothing wrong, whereas plenty of vets from what I have heard are causing distress and upset to new owners when there is no need.