I am not sure that what this breeder is saying will turn out to be correct. If you google retinal detachment in dogs you will find that it can be very serious.
I would hesitate to take on this puppy on the advice of the owner's local vet. Retinal detachment can indicate underlying illnesses or it can progress to complete blindness A vet is unlikely to be knowledgeable enough to be able to say for sure.
If the breeder has taken the puppy to a Veterinary Opthamologist then I would want to talk to that eye specialist or at least see a detailed report.
Are you still expected to pay full price for the puppy?
Cavaliers......Faith, The Ginger Tank and Woody.
Japanese Chins.... Dandy, Benny, Bridgette and Hana.
Remembered with love......... Tommy Tuppence and Fonzi