What you created was a derivative work and thus an infringement. You do have a copyright interest in your translation by operation of law, but this does not mean you have any right to commercialize that material absent a license from the original copyright owners. If, for example, I decided I want to record my own version of a Lady Gaga tune I would have a copyright interest in my sound recording (my version so-to-speak), but would not be able to do anything with that work absent a license.
I might be a good idea to discuss all the specific facts and circumstances with a lawyer in private so you are certain to understand your rights.
Most of us here, including myself, offer a free phone consult so you may want to take advantage of that.
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You're not thinking about this correctly. The basic, operative fact is that you had no right to translate the opera. As my colleague notes, that translation is a "derivative" of the opera --- and only the owner of the copyright in the opera has the right create derivatives of that work. Just like noone but the author of a book can make a movie based on the book. The fact that the copyright owner is willing to work with you so you can use the translated version "for a single project" is a good thing. The copyright owner has no obligation to grant you any right to use the translation. Your ONLY leverage in this situation is that the copyright owner is, apparently, interested in acquiring the copyright in your translation in exchange for granting you a one-time license to use that work. That's better than nothing. Speak with your own copyright attorney to negotiate that deal [if it's still on the table]. Good luck.
The above is general information ONLY and is not legal advice, does not form an attorney-client relationship, and should NOT be relied upon to take or refrain from taking any action. I am not your attorney. You should seek the advice of competent counsel before taking any action related to your inquiry.
You need to see an IP (intellectual property law) attorney who has strong expertise in licensing. There is a cross-licensing opportunity where both you and the music conglomerate make some money. The music house has given you a proposal that heavily favors them because they hold the better position, namely that you cannot legally commercialize your translation at all without their permission. They, on the other hand, can have the well-known opera translated and bypass you. However, you are not powerless. They can save a large translation fee if they like your translation. They are giving you a way to accomplish your project if you give them rights to the translation.
What you need is an expert licensing negotiator to work a deal with the music house whereby the get an the sole license to your translation rather than an assignment of all rights and you, in return, get some small royalty if they use your translation. If this single project is a big money maker for you, then that may be a good consideration for giving an assignment, but you should be trying to work a better deal, I think. You won't likely get a better offer on your own, so take my advice and contact an expert. You probably want someone who is a member of the LIcensing Executives Society (LES), the top trade association for licensing professionals worldwide. I can refer you to one of my fellow members in LES that is near you in CT if you would like.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.
A translation is a derivative work. You technically violated copyright law because you made this translation without approval of the copyright owner of the original opera. You are quite lucky that you have not been sued for doing this, because you could face liability for willful infringement of up to $150,000 together with attorneys fees.
On the other hand, your translation may be quite valuable. You did a lot of work to create this translation, and it could be used for decades to come through-out the world. Although the copyright owner ("big music house") can stop you from using your translation, you own the copyright on your translation and you can stop the big music house from using it. Thus, you have some leverage in this situation. The deal that is being offered to you is unfair----you should get more than one time right to use your translation. Rather, if you "cross-license" your translation to this big publisher, you should be entitled to reasonable royalties whenever your translation is used. You won't be entitled to anything close to the royalty payable to the original composer (or the owners of the original songwriting and copyright publishing), but you are to get something out of this.
You need to retain experienced counsel to negotiate a deal for you. Right now, big music house is trying to walk all over you. They think you are ignorant of the law and that you will give away your valuable translation to them just to get out of this mess. The only way for you to achieve fair value is to retain experienced counsel to negotiate with these people. I negotiate with "big music house" all the time, and they will listen to me if you hire me (I am in New York). They will not listen to you--even with the wisdom that you learn on this web-site. Get a lawyer to handle this for you.
The copyright owner owns the exclusive translation rights as a derivative work. You have copyright in the original elements of your translation, but you cannot exploit these elements as a part of the copyright owner's work without infringing the copyright owner's exclusive rights. By creating the derivative work, you have infringed the copyright owner's exclusive right. Therefore, you will need permission or license from the copyright owner (the big music conglomerate) to use your translation.
You should retain legal counsel to try to negotiate the most favorable rate for your translation fee and see if you can perhaps get a royalty for future uses of your work and the right to use the work in your business portfolio for promotional purposes. However, if you try to play too hardball on your fees, the big music conglomerate can easily find someone else to translate the well-known opera if the conglomerate feels that it needs to exploit this market and it cannot secure the translation rights from you. If it cannot secure the rights from you, the conglomerate will then cease & desist you, and you will not be able to use the translation.
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