Yeah, but technically the stock you sold today hasn't settled, so the funds can't be used for day trading. You could being in a margin account use the funds to do an over night trade. But selling some stock and then buying and doing a sort day trade/over night hold is just to convoluted and as you see ain't allowed. Just doing a straigt day trade, and then later a buy would have been fine, but not mixing it all together.
Cash balance by the way is exactly that, the amount of cold hard cash in your account. So if you have $10,000 in your account and buy $15,000 of stock, you have no cash in your account. You actually owe the broker $5,000, ie your margin debt would be $5,000 which you would be charged margin interest. Now for the reminder of the day you would still have $5,000 in stock buying power left, since you started with $20,000. Of course the next day, if it is a 30% marginable stock and the closing price is what you paid, you would have $11,000 in stock buying power. If the closing price was up/down you would have more/less buying power the next day.
And if you short stock, your cash balance goes up, since you sold stock and got cash for it. And Datek will pay you interest on that cash. But no, your buying power doesn't shot way up with all that "cash" balance in your account. The margin you get on short stocks is figured different from long stocks, and sort of works out to about the same.
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