According to the opening paragraph of the Wikipedia article for "Futures contract", the parties to a futures contract
initially agree to buy and sell an asset for a price agreed upon today (the forward price) with delivery and payment occurring at a future point, the delivery date.
However, later in the same article, a formal definition of a futures contract is given, attributed to Björk, which states that, if $F(t, T)$ designates the quoted market price at time $t$ of a futures contract for delivery of $J$ at time $T$, then, in particular,
At time $T$, the holder pays $F(T,T)$ and is entitled to receive $J$. Note that $F(T,T)$ should be the spot price of $J$ at time $T$.
So, I'm confused: is the amount to be payed on delivery date in exchange of the underlying asset determined on the day that the futures contract is entered, as the opening paragraph indicates, or is it determined by the market quote on the day of delivery, as Björk's definition indicates?