In a Nutshell: It is permitted by the Hanafis to buy and sell dogs for a lawful purpose, e.g., hunting, guarding, guiding the blind and so on. Imam Malik permitted it regardless however the other schools of thought prohibit such buying and selling.
All however allow a price to be given for relinquishing control of a dog - which differs from a sale in that the one relinquishing control does not carry the usual responsibilities associated with a sale such as quality, fitness for purpose, delivery etc.