I think the breeder sounds truly concerned for the cat and that is why she wanted him back. She wasn't, nor can she, force you to give him back. (Unless you were the original owner and it was in contract, which does not involve you) I think she is worried about his health and just wants to be sure he is ok. Its also true that other things can be done to figure out the coughing thing and wheather its related or not. As far as I know a good chest x-ray can even help in determining if he has fluid build up or an enlarged heart. Im sure you will make the best decision for you and him.