There isn't a lawyer that could help you with Walmart decreasing your credit limit.
I mean, you can hire a lawyer if you want, but he won't be able to help you. If you look up in their terms and conditions (and just T&C for all unsecured credit cards really), you'll see they're allowed to decrease your credit limit on a whim, and don't need to give out a reason either.
That being said, something obviously happened, so you might want to call them and ask politely. They might be able to help clear this up.
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