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As an attorney, I would always want a little bit more language in a contractual release. The key is that the parties need to agree and understand the release to be contractual in nature, supported by "consideration" (a bargained for agreement), and it should be clear and comprehensive to the issue at hand. I would say a "better" release would be the following:
[Their name] fully releases, acquits, and forever discharges [your name] from any and all claims, actions, causes of actions, grievances, obligations, rights, losses or liabilities of whatever nature, whether known or unknown, disclosed or undisclosed, asserted or unasserted, in law and equity, contract
, or otherwise, including without limitation, any claims arising under federal, state or local law, and any claims arising out of any relationship between [your name] and [Their name], including but not limited to any claims or counterclaims
that were or could have been asserted in the Pending Case and/or matter. Further, [Their name] intends that this Release shall fully discharge the [your name] to the maximum extent permitted by law.
Now there's also a trade-off, in that if it's more informal like what you put, that would make it more likely that they would agree to it. If you use my suggested language, it's less likely that they would agree (but it would be more likely to hold up in court should this actually go to court). I tried to keep it short as possible (because releases can go many pages)... but you do need to understand that possibility.
Finally, I would suggest investigating this on your own. I find it strange that they would contact your buyer about a "copyright" issue. This isn't copyright. It's trademark law. It could also be "cybersquatting" under certain instances, but it doesn't implicate copyright. So it's possible that the website contacted your buyer but is not serious (in that they have not gotten a lawyer yet) or otherwise don't understand the distinction. That's another thing to consider.
Hope that clears things up a bit. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable. Please note that I don't get any credit for my answer unless and until you rate it a 3, 4, 5 (good or better). Thank you, XXXXX XXXXX luck to you!