Since you've already committed to not working with the company again, why not call the highest contact you have and, in as pleasant a manner as you can muster, politely request payment for services rendered? If it were me, it would go something like this:
"Hi ____, how are you?. I understand you've decided to implement the _______ project internally, based on the spec I wrote. I'm really glad it met your needs and I think it will be a great solution for you. I spent ___ hours working on it so I'd like to bill you $____ for my time. Does that sound fair to you?"
Then don't say anything else at all and just listen to his response. No justification or anything like that. He's either going to say yes or no.
If he says yes: "Great, I'll send the invoice over. Let me know if you need any clarification on the spec."
If he says no: "I'm sorry to hear that, good luck with your project and have a nice day." Then hang up and don't ever, ever take a call or answer an email from them again. Period.
If they say no to payment, I'd consider going to their competitors and offering to sell them the proposal as a spec as well. It may also be worth publishing it on your website.
i try to go down that track now - decided to bill them for business analysis and see how things turn out ... i'll report here :-)
billing only for the hours would be under value i think, i simply see it as a business analysis and even offer them to give them deeper insight in their own business if desired - simply change the seats ... lets see.
That's why sometimes spending too much time just talking to the customer, outlining and drafting some spec just isn't worth it. Get on the hourly as soon as possible and then take as long as the customer feels it's necessary to have the perfect spec.
Spending some time getting new projects is part of the cost of doing business but producing useful artifacts (be it code, designs or a specification) for free isn't good unless you have a really good relationship with the customer.
This can happen in all walks of life. My neighbour is a very good carpenter, made a really nice cad drawing + materials bill for a local church, they took it to a competitor to have it built.
Limiting your exposure and building up a portfolio of 'good' customers is a slow process, it can take quite a few years to get to the point where you no longer have to worry about stuff like that.
The lesson is to not give too much to a customer before you have established under what rules your relationship operates, this customer saw you do a lot of work that others would charge for for free and took advantage of that.
They felt they were not under any obligation to have you do the work, it's unethical of them but it is also partly your problem because you gave them that opportunity.
If you can avoid giving customers detailed specifications - and I really don't see how that is a requirement before getting to a contract, a global spec that you could hand to them should suffice, not something they can start building off - then that would probably be enough to safeguard from instances like this.
It's probably happened to all of us at least once though.
Also, to every story there are two sides, I'll take it on faith that this is how you see it, but chances are that they see it completely differently. They might think they guided you in drawing up the spec. Perspective can make a big difference in cases like these.
Next time be more brief, put enough time in to a proposal to make clear that you are serious, not enough to give away a serious chunk of work.
edit: on another note, I think I can glean from your writing that you are a German national working abroad. Cultural differences about stuff like this abound, not only would you probably have had a case in Germany, your customer likely would have never done that. Western Europe has a different attitude towards stuff like this than most of the rest of the world, I've found. Verbal agreements being honoured is a pretty rare thing elsewhere, it's all about the contract. I've lived in Canada for a good five years, last year I was in Germany for some business, and the company I visited gave me a roughly 7,000 Euro computer + CNC controller on my say so that I'd pay when I got back to NL, saving me a trip. I could never imagine that happening in Canada with a new business relationship.
You may have to do 'in Rome' as the Romans do, or you'll be taken advantage of!
That's pretty balanced. If it were a lot of money most people would go to speak to a lawyer, but when legal action starts, the only real winners are the lawyers.
I second the suggestion that you contact someone as high as you can, call them, and tell them that you're pleased they liked the work, that you spent X hours on it, so would like to bill them $Y, then shut up.
It's good advice, in another comment in this item, and here's the link.
Better than one experience I had years ago when we gave a long and very detailed proposal only to find out later that it was a guy doing research for his MBA.
Good customers offer to pay for detailed proposals like this. They only ask for a few free hours of your time in order to meet you and see if they want you to make a proposal. Great customers will offer to pay you for even that time.
The situation changes a bit when you deal with huge companies or governments that send out requests for proposal and expect a lot of work up front for no pay. But that's because those contracts tend to run in the high six figures to millions of dollars realm and they want to attract bigger companies that can afford to spend a considerable time making a proposal without being paid.
Strike it up to a learning experience. The next time you want to make a proposal, outline to your client that you bill for your time (or pitch a fixed cost). If they decide to go with you after the proposal though, you'll treat that fee as the deposit so it acts like a discount.
Legal action only makes sense when there is some substantial amount of money on the line. Only punters waste their time suing someone over the theft of a proposal. If the proposal contained so much work that it could lead to lawsuit, the author should have provide a summary version with an option to receive a full version upon payment.
This kind of thing happens all the time when you put your best work on the line. I've always felt that you're better of putting in the effort up front and get left at the alter from time to time than you are to withhold the effort. It's a matter of enlightened self-interest. At some point, you will find a client who is tired of all the crap vendors they've worked with in the past and will pay you for what you're worth.
"Hi ____, how are you?. I understand you've decided to implement the _______ project internally, based on the spec I wrote. I'm really glad it met your needs and I think it will be a great solution for you. I spent ___ hours working on it so I'd like to bill you $____ for my time. Does that sound fair to you?"
Then don't say anything else at all and just listen to his response. No justification or anything like that. He's either going to say yes or no.
If he says yes: "Great, I'll send the invoice over. Let me know if you need any clarification on the spec."
If he says no: "I'm sorry to hear that, good luck with your project and have a nice day." Then hang up and don't ever, ever take a call or answer an email from them again. Period.