Parseltongue isn’t Unbreakable Vow, it doesn’t prevent people from changing their mind. Any attempt from Harry to abuse the promise like that will probably make Voldemort reconsider and no longer allow to name new persons for the same “class” of powers (like, early in the year, he said things like “no more body parts” when Harry was enumerating lots of body parts he could use to kill someone).
Parseltongue isn’t Unbreakable Vow, it doesn’t prevent people from changing their mind. Any attempt from Harry to abuse the promise like that will probably make Voldemort reconsider and no longer allow to name new persons for the same “class” of powers (like, early in the year, he said things like “no more body parts” when Harry was enumerating lots of body parts he could use to kill someone).