Change the SqlConnection,Credentials property

So i have a login page and i have to implement a validation: 'If the user is "sa" then it should connect to the Database using that users credentials', which means i have to modify the credentials of my SqlConnection object:

        if(username.Equals("sa", StringComparison.OrdinalIgnoreCase))
        {
            var securePassword = new System.Security.SecureString();
            foreach(char character in password)
            {
                securePassword.AppendChar(character);
            }

            securePassword.MakeReadOnly();
            var credentials = new SqlCredential(username, securePassword);
            sqlConn.Close();
            sqlConn.Credential = credentials;

            sqlConn.Open();

            return true;
        }

But im getting an exception in sqlConn.Credential = credentials; even though the property is not readonly

InvalidOperationException: Cannot use Credential with UserID, UID, Password, or PWD connection string keywords.

Is there any other way to change the Credentials property?

Thanks in advance.