Inverse bitcoin contracts are usually denominated in USD terms. So if there's a contract value of $100 then each side -- LONG and SHORT -- puts down some portion of the contract value in margin. The contract is an agreement between traders to pay the other side an amount of bitcoin profit/loss as the price changes. Each contract has an expiration date, and some exchanges have periodic (daily or weekly) dates where the contract period's profits are "settled" even if the contract has not expired.
How can this be? How can you have more futures contracts for gold than actual gold? Because you don't have to deliver a bar of gold when the contract matures. Many futures contracts settle on a "cash" basis – instead of physical delivery for the sale, the buyer receives the difference between the futures price (= the agreed-upon price) and the spot (= market) price.

Exchanges simply take a fee to facilitate the orderbook where its clients (the counterparties) create and trade futures contract with each other. They also have to manage the system's risk so that traders don't get overleveraged or manipulate the market. Since counterparties are only putting margin down that is a % of the contract value, the exchange also has to handle liquidation procedures in case the value of the margin is exceeded by the loss on the notional market value of the contract. For instance, BitMEX offers 100x leverage, so if you want to enter a $10,000 position you need to put down $100 worth of bitcoin. If the price moves just 0.5% against your favor, BitMEX will take over your position and execute it into the market, so that the person on the other side of the contract can have someone else who pays for the profit. In the event that the liquidation doesn't get passed off to another trader, an Auto-Deleveraging/Termination can occur, or Socialize Loss in the contract builds (we will discuss these issues in more detail later).

Recent bitcoin futures contract announcements from CBOE, CME, and Nasdaq have generated tremendous interest in digital assets. Bitcoin futures have been highly anticipated as they will provide traditional financial institutions with one of the first opportunities to meaningfully participate in the digital asset space via a regulated investment framework. It is an opportunity for Wall Street to catch up with Main Street on bitcoin. With the impending launch of U.S.-listed bitcoin futures, investors may wonder what the bitcoin futures curve might look like. Using information from existing digital asset derivative trading platforms such as Bitmex, OKCoin, CryptoFacilities, and BTCC (all exchanges outside of the Commodity Futures Trading Commission purview), MVIS Research has constructed an approximate curve based on non-U.S. bitcoin futures trading on these exchanges. These are real trading platforms revealing real volume.

Since you bought 68.4246 BTC, you want to short 68 BTC and lock in the USD value. You will have 0.4246 BTC left over, which will give you a slightly long bias. You can either fix this by only buying 68 BTC in step 1 or by giving yourself more short exposure by shorting 69 BTC. Remember that CryptoFacilities contracts are inverse (which allow locking in USD) and are denominated in BTC