Is this a breed that is known to have heart issues? I suspect that it is, if the breeder is stating that they do "cardiac clearances". A true OFA Cardiac Clearance cannot be given until the dog is 12 months old. At this point, the breeder is UNABLE to provide a cardiac clearance certificate to you, since you reference a "puppy".
The fact that the breeder said that she sells puppies who have "cardiac clearance", when it's not even possible until the dog is 12 months old, should have been your red flag that you weren't going to get what you were paying for. It's similar to buying a breed that is prone to hip displaysia, and having the breeder say they have 'hip checks' done. In most dogs, there is no way to 'check' a puppie's hips and know whether they'll suffer from hip issues as an older dog. Same deal with the heart 'clearance'.
Finally, a heart murmur CAN be congenital. It can also be developed later in life. There's no guarantee that at a previous appointment, a vet would have detected a murmur.