I just got a 7 month old Havanese puppy Saturday June 18th. Breeder said I get 72 hours to have him checked out, which I did, and found out that he has a level 2 heart murmur. My husband feels and so do I (however he is such a adorable little guy) that I should return him to the breeder.
What does your vet say? Personally I wouldn't return him but I would ask the breeder for assistance in paying for an ultrasound to determine exactly what you are dealing with. I might even ask for, at minimum, half your purchase price be refunded. A heart murmur is not necessarily an immediate death sentence. Dogs can live years without it getting any worse, etc. But I would definitely suggest an ultrasound be done soon.