Praise be to Allah.
In my opinion this is not permissible, because dealing with them usually only comes after getting close to them by means of things that they like. This is the action of the practitioner of witchcraft who gets close to the Shaytaan by doing things that he likes, which cancels out all his good deeds. When he mentioned that he uses mercury, this is deceit and taking peoples money under false pretences, and it may come under the heading of gambling and stealing. The shayaateen work by illusions and magic, and it is known that they do not possess any kind of money or currency or goods. If they give anything of that sort to a person, either it is an illusion that is not real, or else it is something that has been stolen from other people. It is not right to deal with them at all.
In fact, many of these fraudsters who claim that they can make money are cheats and liars who deceive the people and destroy their wealth and efforts; it is no more than theft. The newspapers are full of stories like this. We ask Allaah to keep us safe and sound.