Then if you have a house, sell it at a high price and buy it back at a low price, you still have a price difference.

You must understand what you are trading, not futures contracts. It is equivalent to signing a contract. He buys high and sells low here, which means that you buy a put contract at a high level, which means that you are bearish at the price of 10000 yuan, and some people are bullish at the same time, so you two sign a deal at 9000 yuan, you are the seller and he is the buyer. He is bullish and certainly wants to buy at a lower price. Then it really fell to 9000, and your contract came into effect, which is equivalent to selling the contract.

Your business here refers to the business contract, which is different from the right that the contract gives you to buy and sell physical objects.