The confusing part is that its the second sale that causes the problem, read this and see if it makes sense: http://www.thestreet.com/markets/ericgillin/10071060.html
If your trading on a cash account, it takes time. I dont know why sense all this stuff is susposed to be computerised, but in my experiance, you can still buy more stock with any funds you have left over, you just cant sell them till 3 days after you buy. - nathan