Asus or Abit? Which is it?
You might try being upfront and tell the customer that you have just discovered that a very minor component that you have been using in systems may only be of marginal performance, and that you would like to exchange theirs for a better one, and do it for no additional charge ... the MSI's are only about $11 and it sounds like your worried about it, so the cost to you would be well worth the easing of your worries.
Did you sell an overclocked system to someone? I would not recommend doing that, ever. It's hard enough providing warranty service if you are a small time "builder", which is why retail parts are the only way to go (because they have good warranties that you can rely on). If a "customer" wants to overclock, let HIM do it and potentially void the warranties, not you.