What amount is subject to backup withholding with respect to security sales made through margin accounts?

The amount subject to backup withholding in the case of a security sale made through a margin account is limited to the amount of cash available for withdrawal by the customer immediately after the settlement of the sale. The amount available for withdrawal by the customer does not include amounts required to satisfy margin account maintenance. If a margin call forces a customer into a sell-off, such proceeds are not subject to backup withholding.

For additional information see Publication 1281, Backup Withholding.