What is the ruling on working in dog training, given the impermissibility of selling them?

Originally, keeping dogs is religiously forbidden, and it is only permissible for a need such as hunting, guarding, agriculture, or livestock. It has been authentically reported from the Prophet, peace and blessings be upon him, that the reward decreases for whoever keeps a dog for purposes other than these.

The ruling on training dogs is a subsidiary of the ruling on keeping them. If the training is for permissible purposes such as hunting, guarding, or for tracking criminals, or in warfare against the enemy, or any private or public benefit, then it is religiously permissible, based on the Almighty's saying: "They ask you, [O Muhammad], what has been made lawful for them. Say, "Lawful for you are [all] good foods and [game caught by] what you have taught of hunting animals as trainers, teaching them of what Allah has taught you. So eat of what they catch for you, and mention the name of Allah upon it, and fear Allah. Indeed, Allah is swift in account." (Al-Ma'idah: 4), and because necessities make forbidden things permissible.