There is no such letter from the FDIC confirming 40bn of assets which belong to WMI.
Otherwise why would the FDIC say this on their website:
Receiver does not currently have and does not anticipate accumulating sufficient assets to pay in full all of the allowed claims of the general unsecured creditors of WAMU. As a result, the Receiver does not project having sufficient assets to make any distributions to WAMU subordinate note holders or equity holders. Therefore, it is unlikely that any distributions will be made to WMI or its successor on its claim as sole equity holder of the failed bank.