According to the opening paragraph of the Wikipedia article for "Futures contract"
In finance, a futures contract (more colloquially, futures) is a standardized forward contract which can be easily traded between parties other than the two initial parties to the contract.
When a futures contract that has already been established is traded,
- Does the delivery price change?
- Does the contract have a non-zero value?
- What happens to the margins accounts?
To elaborate, suppose at time $0$ I take a long position on a futures contract, $C$, on one unit of an underlying asset $A$ to be delivered on time $T >0$ for the delivery price of $P_0$. Suppose at time $0 < t < T$ I wish to sell $C$. It is my understanding from the quote cited above that it is possible to do so. Assume an arbitrage-free market with a constant zero risk-free interest rate.
Suppose at time $t$ the market quote for futures contracts on one unit of $A$ to be delivered at time $T$ is $P_t$. Which delivery price is the counter-party to whom I sell $C$ committed to? $P_0$ or $P_t$. In other words, suppose the counter party to whom I sell $C$ holds $C$ till the time of delivery. Are they required to pay out $P_0$ or $P_t$ in exchange of the underlying asset?
Is the counter party I sell $C$ to required to pay me for $C$? In other words, at time $t$ does $C$ have a non-zero value? We know that at the time of delivery, $C$ has a non-zero value, namely $S_A - P_0$, where $S_A$ is the value of $A$ at time $T$. This must be the case, since otherwise there is an arbitrage opportunity in the market. Indeed, if $C$'s value was naught at time $T$, a trader could buy $C$ for free and immediately parlay it for the underlying asset in exchange of $P_0$ for a net wealth increment of $P_0 - S_A$.
When I sell $C$ to the counter party at time $t$, what happens to my margin account? Does the counter party has to set up their own margin account? Does it matter if the exchange takes place before or after that day's end of trade time?