There is a $500 initial minimum balance required to open a cash account.
There is a $2000 minimum equity requirement to initiate margin trading. The requirement may be met with cash or marginable securities. In a cash account the anticipated dollar amount of the purchase must be deposited before an order will be accepted.
If a withdrawal request is made that would bring an account's equity level below $500, it shall be considered an intention to close the account. Therefore all securities would need to be liquidated (or disposed of), or transferred, before the cash withdrawal request would be processed.