It may be a work for hire agreement. Have a local attorney review your contract, emails and any other correspondence. Good luck.
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It would depend upon whether you assigned copyright, or if copyright vested in him by contract, or if you reserved copyright. I suspect that it IS copyright infringement, without knowing more.
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Did you have a written employment contract? Were you an employee or an independent contractor? I suspect that there are more facts than you listed in the post. You might benefit from a consultation with an experienced attorney.
There are always at least two sides to a dispute. I'm quite sure the person who hired you will say that you failed to perform as promised and that his decision to pay you less than the agreed-upon amount [assuming it was agreed-upon] was justified. In fact, he may claim that because of your failure to perform he had to hire a different designer and that YOU are liable for that extra cost.
You want to know if you have any leverage that can be used to extract from him the amount he withheld. If that amount is less than $5,000 then pursuing this matter through legal counsel will not be cost effective -- even if you do have leverage [through copyright or contract or any other type of law]. Some disputes, and most inequities, have no efficient remedy or, indeed, any remedy at all.
There's a wrinkle in your fact pattern: You note that the website you created displays photographs and plays videos. You need to make sure that whoever owns the copyrights in those photographs and videos have agreed, in writing, that each one can be shown and played via the website. Without those permissions, YOU are likely liable for the infringements [along with your former client, the website owner]. If that situation is possible then you need to immediately speak with your own intellectual property attorney. Good luck.
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