When you place a trade for whole shares when the market is closed, you are creating a Market on Open order that will process the next business day when the market is open. Due to after hours and premarket trading that some firms have available, the value of that stock can fluctuate by the time the market opens and the Market on Open orders are processed. Before the market opens for the day, our system will check the current price of the stock on your buy order. If the value of that stock went up and you do not have enough buying power to make that purchase, our system will cancel the order. Once the market opens the Market on Open orders will start processing in the order they were received.
You may have enough money in your buying power when the market opens, but due to market fluctuations and the time your order was received for execution, the price of that stock may change as the orders are filled. Your purchase will fill at whatever price that stock is currently trading. If the price of the stock has gone up and you do not have enough buying power, it will overbuy, the order will be filled, and the order will not be canceled.
If you have a negative buying power due to a Market on Open order overbuying in your account, you will want to make a new deposit or you can sell some of your investments to cover the negative buying power.