I do not see there is a problem to stay longer with the breeder either.
My puppies usually go home at least 10 to 12 weeks...smaller ones stay longer.. if I am getting a pup from a breeder,I prefer get my pup at 3 and 3 half months not at 6 months old so they can get used to my son handing them,his fast movement...
Is your puppy smaller ? maybe this is the reason why the breeder wants to keep the puppy longer!