GENEVA, Jan 30 (Reuters) - The U.N. chief has told states that the organisation is at risk of "imminent financial collapse", citing unpaid fees and a budget rule that forces the global body to return unspent money, according to a letter seen by Reuters on Friday. "The crisis is deepening, threatening programme delivery and risking financial collapse. And the situation will deteriorate further in the near future." The U.N. is facing a cash crisis as the world body's largest contributor - the United States - has slashed voluntary funding to U.N. agencies and refused to make mandatory payments to the...