You can sue for the money. There was an agreement, even if only an oral or unwritten one, that you would travel and work in exchange for a certain wage (and being paid for all hours worked) and a per diem. If you were not paid as agreed, your employer breached or violated that agreement; since oral agreements are enforceable, you could sue for breach of contract for the money to which you are entitled. To win, you'd need to establish in court the terms of the agreement and that you did the work, by some combination of testimony, documents (like emails, text messages, plane tickets, etc.) You can also sue for your flight back, since the employer should have paid.