First and foremost, you should be aware of settlement cycles. You are not required to have stocks to sell in the market because the expectation of the market is that when the buyer wants the stock, you would have completed your pay in. So, you have some time a day in best case to make up your mind on getting the stock either at a higher rate or a lower rate and pay in or go the standard defined route of shortfall.
Secondly, a typical short seller is neither interested in buying the stock at a higher price nor going in for a shortfall scenario. He is interested in making a profit from the market movement. This means, he is planning to square off the trade before he is asked to make the delivery. So, you are not worried about where the stock comes from, since you are not long enough in the market to go in for securities settlement. So, what remains then is only cash settlement.