My client tells me he will purchase program X for $Y.... Except my client takes the program and doesn't pay because he doesn't believe in copyright, and I've still got the original program, so he hasn't stolen anything.... I haven't lost the program - all I've lost is the ability to economically exploit it. And yet almost all of us will acknowledge I've had my work stolen.
If by "almost all of us" you mean "people who agree with me"... well, that's obvious. Otherwise, no, you haven't had your work "stolen". Not legally, not morally. Unless your client agreed to a contract to purchase the program from you, you never had any claim to the money you expected to sell the program for at the end, just the program itself, the actual product of your labor, which you still have. The just reward for producing something is you have the product and can use it as you please; there is nothing inherently exclusive about the arrangement, particularly when it comes to non-scarce goods like information.
Now, if you did have a contract in place (which would have been the smart way to arrange matters, as opposed to working on speculation) then the promised money became yours when you delivered the program. At that point the client is in possession of your money, and if they refuse to turn it over to you on request then they are withholding your property from you and actual theft has taken place. You don't need copyright for that, of course, just regular property rights and contract law.