If a client won't pay you for your work, then they are almost never worth the time.  I've heard it all before, "I'll pay for the next job," "If I sell any product I'll pay you," etc.  The truth is that if they won't pay for the first job, they are going to expect more jobs for free.  Hagglers will alway's be hagglers, and freebies rarely pay off for photographers. Read it here