Customer accounts have been moved to other clearing firms, but not all positions or monies have accompanied the move and that might have caused accounts that do not have sufficient margin to sell other assets to top off the accounts.
But anything spent on the card will be charged at the customer's normal rate, and any money paid towards the bill will go to clearing the transferred balance first.
By requiring clearing houses to hold more money in reserve, the authorities hope to protect such firms against a major customer hitting a financial crisis.