Unless you arrange for a pre-borrow, you will not be subject to stock borrow fees if you open and close the short position within the same trading session. When a short stock position is initiated, a locate is performed so as to make a reasonable determination the shares are available to borrow, however, the actual borrow does not take place until settlement (T+1 for U.S. stocks). If an account closes the short position on the same day opened, the sale and purchase will net and settle on the same settlement date, with no borrowing of shares required.
In the event you arrange to pre-borrow shares, you are subject to borrow fees at the point of pre-borrow. At the moment pre-borrow is available to Portfolio Margin accounts at IBLLC and IBUK.