As an artist i 've experienced more or less those situations, clients don't want to pay and they try to trick you by saying that they will give you a lot of exposure...The only advise i can give is send a lot of sketches first, let him decide what he likes, send him some more, take the money and then finish the artwork. Don't put too much effort from the start, clients tend to change their minds a lot. I love your artwork , good luck and keep up the good work:)