Assume a future contract specifies that the holder of the contract will need to buy corn from XYZ farm at £100 per tonne, 1 tonne. So total deal value when realizing this contract is £100.
We all know the contract can then be sold in the market before expiry, and the price can fluctuate.
Here's the thing I don't understand: A future contract is essentially a non-optional call option. The contract IS the guarantee that the trade will take place at £100. Assume the last spot for corn is £90 per tonne. You know, once the contract is written, you can't erase the £100 per tonne strike on the future contract. Therefore, the market value of this contract, right before the last trading second, is supposed to be £-10 because the holder of the contract is going to pay £10 pound than the market value to buy corn from XYZ farm. But things like this never happen - future contracts never have negative prices.
So ... which link in the chain of my understanding broke?
Please enlighten me.