Complex sentence: Thomas was approached to fight Khan in December 2005 on the undercard of the Williams v. Harrison but he had contracted to fight Baz Carey, whom Khan had beaten three months previous to that.
Split into simpler sentences: Thomas was approached to fight Khan in December 2005 on the undercard of the Williams v. Harrison. However, Thomas had contracted to fight Baz Carey, whom Khan had beaten three months previous to that.