I think it will depend on how long ago you bought it. At the store where I work, we will give a cash refund (or however they paid) for 30 days, and credit up to 90 days. This is assuming the merchandise is resellable, i.e., the damage is not a result of you abusing it, the broken button would be a manufacturers defect.
Wal mart is usually more lenient than most places, so hopefully you'll be okay. Unless is was a really long time ago that you bought it!