I'm glad you're considering a civil suit. I wonder do you need proof, like if he signed a paper - stating you and he agreed to sell the guitar and you would split the money. I think it would be a good idea, for you have an assertive man you know, try one more time to get the money. You could have the caller say "Hmmm I wonder what your boss would say, taking a woman's guitar doing a service call". Don't tell him you're going to file a civil suit because, it'll give him time to think and if there's no proof, he'll think he has a chance of not paying. Even if you lose the case, it would be worth it, to inconvenience him.