Your personal broker does not make the decision. In fact, many of the limits are controlled by federal regulation. The timing of a margin call or asset liquidation can be very bad. The financial institution cannot risk letting the loss in your account grow if you do not have the assets to cover the it. When you put in an order to short stock you may not get the price you expect.
Regulations require that a short sale can only be done on an up-tick in price.
So, you might put in the order to short only to have the stock trade straight down. This mean you never were allowed to sell. This rule exists to help provide orderly markets and prevent short raids on stocks. There are no special rules surrounding your purchase when you are covering the stock. You should understand that when you sell a stock you do not own.