(a) Advances for customers' claims In order to provide for prompt payment and satisfaction of net equity claims of customers of the debtor, SIPC shall advance to the trustee such moneys, not to exceed $500,000 for each customer, as may be required to pay or otherwise satisfy claims for the amount by which the net equity of each customer exceeds his ratable share of customer property, except that- (1) if all or any portion of the net equity claim of a customer in excess of his ratable share of customer