C# - Display Original Sitecore Username Instead Of What's Currently Typed In?

I might be overlooking something obvious here, but is it possible to return a Sitecore user with the username in the capitalisation they used when registering?

At the moment the username will be displayed as whatever the user typed when they logged in, but I'd like to be able to get the original string.

I'm using User user = User.FromName(domainUser, false);

UPDATE: This is what I ended up with after Yan's excellent answer:

// get the MembershipUser object normally by name
var initialUser = Membership.GetUser(domainUser, false);
if (initialUser != null)
{
// get the same MembershipUser by Id - in this case it retuns username in correct case
initialUser = Membership.GetUser(initialUser.ProviderUserKey, false);
}
// get the Sitecore user from the username in correct case
Sitecore.Security.Accounts.User user = Sitecore.Security.Accounts.User.FromName(initialUser.UserName, false);