If you close your position before the contract expires (that is, give it to someone else), you don't need to pay in cash, so it doesn't matter if you have the goods now. In order to ensure that you can perform the contract when it expires, you need to pay a deposit first. Of course, this is only a part of the contract value, usually around 8%. This is the meaning of futures margin.
Futures itself is a form of contract. Short selling is selling when the contract expires or falls to your expected price, then buying and then selling at the current price in the market. The difference between the two is your profit (and of course your commission).