Is it permissible to use and keep a trained dog to search for truffles, which are sold at a high price, in order to help earn a living, considering that the Prophet, peace and blessings be upon him, made an exception for only three types of dogs?

The majority of scholars agree on the permissibility of owning dogs for beneficial purposes, by analogy with what has been explicitly stated regarding guarding crops, livestock, or hunting. Some scholars have gone further, permitting their ownership for guarding homes and roads. As for purchasing a dog that is permitted to be owned, the majority of scholars have prohibited it, while some Hanafis and Malikis have permitted it, citing the hadith (definition) allowing the purchase of a hunting dog. They argue that a person might need such a dog and not find anyone who would give it to them as a gift.