HARARE (Reuters) - A Catholic priest is mediating a potential political exit for Zimbabwe President Robert Mugabe, although the 93-year-old is insisting he can only be removed via a party leadership vote, political and intelligence sources said on Thursday. Separately, opposition leader Morgan Tsvangirai, who has been receiving cancer treatment outside Zimbabwe, returned to Harare late on Wednesday, a party spokesman said, fuelling speculation about a post-Mugabe political settlement. 