What MF Global did was totally different; > The story states that a London clearinghouse forced MF Global to pay roughly $300 million to back some of its bond

What MF Global did was totally different; > The story states that a London clearinghouse forced MF Global to pay roughly $300 million to back some of its bond holdings during its last week. There's a mountain of difference between sending customer money to purchase customer's own securities, versus raiding customer accounts to pay a margin call against your own bond holdings. You cannot raid customer funds to pay for proprietary (broker's own) activity. Of course using customer own funds to pay for customer's own orders is not restricted. There's nothing to collateralize for a fully cash funded share purchase. Collateral can only possibly be an issue if the customer is trading on margin.