After you place a trade, the transaction is in what is known as the settlement period. What this means is that there is a period where the brokerage works to move the securities and cash between the accounts of those parties involved in the trade (i.e. cash out, stock in or vice versa).
The day that you execute the trade, to either buy or sell, is the trade date. The settlement date is typically trade date + 3 days (i.e. T+3). Your buying power is measured on a trade date basis, where your cash balance is on a settlement basis. If you execute no other trades, your cash and buying power will be equal after the trade has settled.